most modern pop music has 4/4 time now. Every once and awhile you'll get some 3/4 time...and rarely it seems you'll hear musicians stray from that. Tool is still one of my most favorite bands that does.
basically you have to feel the measures/beats and know what they're playing. However, by definition the top number in the time signature denotes how many beats are in the measure...and the bottom number means which note is considered one beat.
So in 4/4 there are 4 beats in a measure and the quarter note gets one beat.
In 3/4 there are 3 beats in a measure and the quarter note gets the beat.
In 6/8 there are 6 beats in a measure and the eighth note gets the beat.
Deciding between 6/8 and 3/4 can be confusing as 6/8 is just a multiple of 3/4 (divide the fraction 6/8 by 2 and you get 3/4). In other words, it's sometimes hard to know if the meter is 6/8 or
a fast 3/4. As you could be feeling two bars of 1-2-3, 1-2-3....when actually those beats you were counting are just eighth notes in a single 6/8 bar and it's really 1-2-3-4-5-6
This is when I like to try and get a feel of the rhythm they are actually playing throughout the song as well as when the chord changes are being hit. This is actually the reason it was put in 6/8 and not 3/4 so that the feel would be different. In 6/8 the eighth notes become more of a set of triplets instead.
4/8 I've never seen as you would just probably write it in 2/4...which you'll probably only see for marches or polkas.
And trust me, it can get much much more confusing than that.