If you can see his rank (on shoulder straps): товарищ сержант/лейтенант/капитан/майор.
If you don't see them (or can't distinguish): товарищ милиционер (some use господин милиционер, but I can't bring myself to say that to them). Some use the standard Извините...
A funny thing is that we would have police instead of militia in a few months and even the Interiof minister still doesn't know what would be the proper form of address.