You could simply paint around the outside edge of the bases - would be faster than applying decals I would think, and you'd be able to quickly tell which one was which. And also, if at some later point, you (or your so n) decide to paint the minis, you haven't really done anything that needs to be undone.
This is actually the reason why I decided to paint my units - I'd never painted minis before, and painted my Dust ones so I could tell the guys apart - now it's become a bit of an obsession and hobby for me!
This is the route I took. On top of that, I've been painting the base of the unit leader a little bit darker for those times when it matters, since I can never remember who's in charge anyway.