At this stage, the examiner will make it quite clear which is which in the way they play the different tempi. However, listen carefully (but bear in mind you won't have long!) as they will always emphasise the first beat. So..
* if it's strong/weak, strong/weak..it's guaranteed to be 2 beats per bar. (2/4 is known as the march tempo. Try and "feel" the left/right left/right pulse in the music)
* if it's strong/weak/weak then this is 3/4. This is known as the waltz tempo, so try and "feel" the 1,2,3, 1,2,3 pulse per bar.
Try and listen to, or play as many pieces of music as you can with these time signatures between now and the exam. That way, you'll soon get to recognise the difference between the two. If you have a teacher who plays piano, ask him/her to play tunes for you too so that you can practice clapping along.
Hope this helps.
Good luck.