It's really a matter of practice.
I used to be quite possibly the worst person ever at these things. It didn't help first year I followed a doctor where we looked at numerous CT scans and he always tried to get me to figure it out.
Doctor: What structure is here?
Me: Uuuuh, the heart?
Doctor: ....we're in the pelvis.
Me: Oh.
But I've come a long way. The best way to think about this is always make sure you've got right and left straight in your head (once again, practice) and just take your time. If you have no idea, start thinking of the things it might be and the things it can't possibly be.