For anyone who doesn't understand the British terminology, by the way, crotchets are the same thing as quarter notes and quavers are eighth notes. Just thought I should throw that out there. And lindley, I suppose you've noticed by now that we're mostly American here, so please bear with us. ^_^;
To answer your question, lindley, it just takes practice. I've always been into math, and since fractions reduce, I always thought 6/8 time was more or less the same as 3/4 time, except 6/8 used more eighths (quavers) and 3/4 used more quarters (crotchets). When I finally started studying music theory in college, I learned the reason for this. It's all about beat placement.
And you'd think that 6/8 and 3/4 are similar, but if you pay close enough attention to the music, you can start to hear distinct grouping patterns (we'll keep using these two signatures as an example). 6/8 is a triple meter, which means the beats will be grouped in threes. You're more likely to hear "DA-da-da DA-da-da" than "DA-da DA-da DA-da" in 6/8 time. The reverse holds true for 3/4 because it's a duple meter, so the notes are grouped in twos. Look closely at your music: you'll see eighths beamed in threes in 6/8 and in twos in 3/4. It takes a bit of practice to hear, but you'll get it eventually. Start with 9/8 time and see if you can't hear, since those beats come in three groups of three.
So what do you do when you have two duple meters, like 2/4 and 4/4? That's a little more difficult, but you can hear the difference in the number of beats per measure. Listen for the stronger downbeat, and that should guide you. The third beat in 4/4 time is also strong, but not as strong as the downbeat. It'll be a bit confusing at first, but you'll learn how to pick it out -- and tell the difference between a third beat and the downbeat of a 2/4 measure -- soon enough. To practice, pick music that has got a moderate tempo; sometimes listening to something that's off at the speed of light will throw your perception and you'll be tapping your foot to halves (I think you'd call those minims) thinking they're quarters.
With something like 2/2 time, I'm sorry, but I can't help you there. I only took two semesters of theory and sightsinging, and I never quite learned how to tell 2/2 from a fast 4/4. But the idea is to practice. Listen to something you've got the music for and try writing it down. Compare your notes with the score when you're done and see how well you do. Try to focus on your common mistakes and you'll get there. Of course, you could always take music theory at university (if you can); it really does help!
I hope I have, as well. Good luck and happy listening! ^_^
