there is a site here that shows population but i think the numbers there are misleading.
it shows characters created not active players on a particular server.
and the feel of your server being under populated could come from 2 reasons:
1) its pre-patch boredom and people have little to do and log in less until the next patch and/or
2) it really is underpopulated.

look at how busy it is during the 1st month of a patch to get a better idea of which one it is