Have you purchased the mike yet? What do you mean by "bad"?
Does "bad" mean "defective" or "doesn't suit my purposes"?
If you haven't bought the mike yet, ask the store to set it up for you so you can play or sing into it and listen to the results over headphones. This will give you a general flavor of what different mikes sound like.
If you already own it, unless you have some technical skills and access to a laboratory, the only testing you can do is to use it and see if it records what you put into it. Make sure you aren't overdriving the mic or preamp, listen for a reaonable amount of volume and no distortion.
It would be best if you had a similar mike (borrow one) to compare it to.
As far as checking a pair of mics for stereo match, you would need a laboratory and quite a bit of analog savvy to set up a test and interpret the results. I suspect what the pros do is to inject a known signal level into both mikes and record the output response of each mike. By sweeping the input signal through the frequency range and plotting the output reponses, you get a picture of how the mikes respond to different freqs. If the frequency response curves match throughout the spectrum (+/- TBD dBs) they are considered a matched pair. They probably track phase shift for each mike throughout the test as well.
I would probably just buy a matched set from a reputable dealer and assume they had done the testing. I think the manufacturers usually supply the results of their testing when you buy a matched pair.
