Red comb, and "loose" vent. A chicken that is not laying will have a drier, tight vent. But I agree, the best way to tell is to separate the bird and see if they lay an egg. The egg will already be in the chute, so pulling them from the pen will not disrupt them at first.
I don't necessarily agree with the other link... you have to be in tune to the girl's molt schedule to use that method, IMHO. But production slows down after a couple of years, so if you're in it for the eggs, just replace the flock every year, or every two years so that you always have fresh layers, no matter how well they do/don't lay.
Just my thoughts on it.