Sometimes they can be sneaky, and lay when you aren't looking.
A few ways to recognize who might be laying:
--layers have bright red combs and wattles, non-layers have paler combs & wattles that are also more pale in color.
--if you look at their vents, a layer will have a larger vent that looks like moist and stretchy (so an egg can come out), while a non layer has a smaller vent that looks dry and puckered.
--you can feel for the tips of the two pubic bones, below the vent and a little to each side. On non layers, those tips are just below the vent and close together. On layers, they are further down and much further apart (again, makes enough space for an egg to come out.)
I find that comparing birds with each other makes some of the differences really obvious. For the vent and pubic bones, a rooster or a partly-grown chick are obvious examples of birds that are not laying.