Yep - very simple. However, it can be a slightly trickier when they are young. With adult fish, males have an orange-red throat/ventral area, extended fin rays and a longer dorsal fin. Younger fish, or fish stressed at your LFS, may lack the bright orange and, if young, won't have the extended fin rays. They will have a longer and pointier dorsal fin, instead, however. You can also tell, by looking at body shape, which are males and which are females. females are deeper-bodied and look wider if you look down at them from above when compared to the slimmer, more stream-lined males. The 'fatter' bellies in the females become even mroe pronounced at maturity as the fish fill up with eggs. Also, mature males are larger than their female counterparts.
I'll look for some good pics on google.com and link to them in a minute
Here's a pic of relatively mature fish at an LFS. See if you can spot the males... Note that they are not as brightly colored as they would be if they were comfortably settled into their tank:
http/www.aquarticles.com/images/Fish%20S...ourami%20d6.jpg
A pair of them:
http/www.aquahobby.com/mleeri.html Note that females do not always have white throats and can often have quite an orange coloration (though not compaired to the males) and quite dark olive coloration.
A selection of pics:
http/badmanstropicalfish.com/gallery/antiban/leeri.html First 2 are both males - note difference in color. The next is of a stunning pair. The next is of 2 females and the one after that another pair.