One thing to keep in mind is that classical notation of time signatures are relative and subjective. This means that if you don't have it in written form or if it hasn't been decided or agreed upon by yourself or the people you are playing with, then it can be interpreted in different ways, to some degree. This is because, in its basic form, time signatures are like fractions. For example, 4/4 time, can also be written as 2/2 time or even 1/1 (although not common). If you are not worried about writing it down then you should focus only on the number of beats. To keep it simple, most music is in 4 beats per measure. As mentioned before, try and determine where the accents of the music are and then count to 4 for each pulse or beat that you feel. If the music doesn't seem to fit in 4 then try counting in groups of 3, since that is the next common group of notes. This is like a waltz feel.
Then, to make things more complicated, you might have to consider the subdivision of each beat. Normally the subdivision of the beat is an even number like 2 or 4, but in some feels the beat is subdivided into 3, like a blues shuffle.
So, in general:
1. Find the beat or pulse of the music,
2. Determine the accented beat or pulse,
3. Determine whether the beats group together in 4 or 3,
4. Determine if each beat feels subdivided into 2 or 3.
Do you have an example?