Coming into this thread as late as this, I'm sure someone has already mentioned some of the things I'm about to mention. But, coming from a different person's brain, things can be explained differently and maybe that different perspective can make it clearer or easier to understand.
First, it is very true that you can't test electricity for the issues that hinder recording by looking. You also can't tell my just plugging something in. But, when looking at an older house, and particularly a farm house, you can spot things that might spell out trouble. Someone may have already fixed it, but if not, the trouble might be something you'll have to deal with yourself.
As has already been said, the code these days is to have the hot wire always on the same prong, the neutral on the other and an earth ground is now the standard, too. Back when everything was analog, circuits were a lot more forgiving and able to withstand some fluctuation in the AC line. This meant you could actually have incorrect wiring (to a point) and everything would still work. Then, you had to deal with AC giving you ground hum and phase buzz that didn't really show up unless you had an amp with speakers. And depending on the outlet you plugged the amp into, you might not even hear these things. And if you happened to have a compressor from a refrigerator or air conditioner on the same circuit, those surges when it went on and off really didn't bother things that much. So, in the old days of analog, wiring wasn't that much of a problem.
But, in the digital world, wiring really has to be correct, or it can eventually damage your digital equipment. The hot wire must be correct,and then the neutral must be correct. And the earth ground is also necessary. It's sad, but many homes built before these standards were in force still have the old analog standard and people can purchase digital devices and find that they don't last as long as they should. A computer on the same circuit as a room air conditioner can die a death too soon, because that compressor keeps going on and off, causing spikes and surges to pummel the computer. People assume everything is OK, as long as everything turns on. Electrical issues can be hidden and unless you are aware of them, your expensive equipment is doomed to an early death.
When I first moved into my house, I didn't know it was built in 1939. Back then, the electrical code was pretty loose. There was just a two wire system and in many cases, electrical tape wasn't even necessary. Wire nuts hadn't been invented yet, so wires were just twisted together with pliers. The two wires were passed through studs and joists by drilling holes and this was how the two wires were kept at a safe distance apart. In many cases they were completely separate wires and if they were together in a jacket, the jacket was made of paper impregnated with tar.
I found some of those wires that had been twisted together with pliers were no longer twisted as tightly as they should be. There were times when I would lose power in a room, and that power loss didn't throw a breaker. I had to get up in the attic and jiggle wires to see which connection was at fault. That told me I had purchased a potential electrical nightmare. If one connection could become loose, what would keep others from getting loose? The sad answer to that question is nothing.
I found that the wire itself was good #12 wire, but it was the connections that were suspect. And there was no earth ground in the house at all. Someone had replaced all of the outlets with the new 3 prong type, leading people to assume there was updated wiring. And because the former homeowner's father had done much of the changes, some of the hot wires were incorrectly connected to the neutral prong of the outlets.
So, I had incorrect wiring, no earth ground and some connections coming loose. No wire nuts or electrical tape was a big issue for me. I didn't like the idea of the two wires (hot and neutral) to be so close together without anything to keep them from touching. That's just how it was done back then, but we now know these things are a problem waiting to happen. Why play with fate?
So, I decided to rewire my whole house. In the process of doing that, I found that every wall outlet in the house was on one breaker. It didn't matter what room that outlet was in, it was on that same breaker. I was beginning to wonder how this house had passed electrical code before the sale. I'm sure there had been some underhanded money changing hands somewhere. Also, all of the wall outlets were about 3 feet from the floor. That was how things were done "back in the day". New methods have outlets about a foot from the floor. So, if you see outlets that high, that should also tell you a little about the age of the house and maybe the age of the wiring.
I finally got the wiring all finished...new #12, 3 wire Romex, to go with the new outlets that were already installed. I wired each room to it's own breaker and in some cases put different appliances on their own breaker, such as refrigerator and furnace. It was a slow job, but because I was putting each room on it's own breaker, it allowed me to have the rest of the house turned on while working. And I have an education in electricity and electronics, as well as being taught by my dad when I was a kid, so the various sources of my training all came together for this project.
This was back when I purchased this house over 35 years ago. I had plans on making one room a studio, so I made sure to put some outlets on a different breaker than others, so lamps and amps and other things wouldn't affect the recording gear. Simple things like a rheostat on the ceiling lights could cause nasty noises in a recording.
Things to look for in an old house. Check the wiring. Find out when the house was built, if you can. That will tell you a lot. And if it's been remodeled, don't assume everything was done to code. Check it, or have someone check it for you. It's already been mentioned that you can get a cheap wiring tester that plugs into an electrical outlet to see if something is wired incorrectly. And don't assume because one is wired correctly that every one is wired correctly. Even the smartest human makes mistakes. You can only hope a dyslectic electrician didn't wire your house.
If you find out the house was old, you need to do some more checking. Get up in the attic and check connections up there. See if the wire is Romex or if it's just single stranded wire. Check for wire nuts and electrical tape. If there is tape, check to see if it's the sticky kind or is it the newer plastic coated kind. That helps to tell you if the wiring has been updated recently. Old electrical tape was sticky on both sides.
Now, if you find any of these things, all is not lost. It just tells you you will need to make sure the line is clean enough for installing digital equipment. Take it from someone who's been through it. It can be a small nightmare, but it isn't always the case. If the house has been remodeled recently, hopefully they also remodeled the wiring, too. If they were smart, they did.
I built the studio myself and did all the electrical myself. The grounds from both circuits go back to the same ground bus in the breaker panel. They are literally right next to each on the ground bus, yet I get a hum when I go to record the keyboard. My workaround is to use an extension cord to plug the keyboard into the same outlet as the interface whenever I go to record the keyboard. It's not often, so not too inconvenient.
. This is what killed not an insignificant number of musicians. The plug is inserted with chassis hooked to line (hot or what ever else you call it), the guitar cord or microphone carries that chassis connection to the guitar or mic. touch either and stand in water perhaps, and BANG, you are dead. i always caution newbies finding some real old radio or amp, know what you are doing when you mess with those.
getting across 120V can definitely kill you if you are on a good ground, but most time it just knocks the shit out of you. 240V is lethal much more often.