If you can catch them (maybe after dark?) feel between the pubic bones over the vent area. If you can get two fingers between the pubic bones, the hen is probably laying. Also, as was mentioned above, the vent will look more full and moist if a hen is laying. That is harder to judge though I think, unless you have had a chance to look at a lot of chicken bums!
p.s. I learned this a while ago while I was in 4-H studying for the national poultry and egg judging competition in Louisville. Seems to work. I have two hens right now, an EE and a RIR. I know the EE is the one laying since the eggs are green and when I compare pubic bones there is an obvious difference between the two.