There are a few ways to do this. First of all, depending on the age and breed of your chickens, it's possible they're all laying- one egg every 2 or three days isn't uncommon. However, if that's not the case, here are some things to look for in laying hens:
- Red comb/wattles- although combs and wattles will stay reddish if the bird has just stopped laying for the winter, if they haven't layed before then only the layers or soon to be layers will sport this color
- Moist, healthy vents- the vent of a non layer will be visibly dry and feathery compared to the vent of a laying bird. You can also check for rigidity of pelvic bones, I think.
- Color of skin- If your birds have naturally yellow skin, then the skin of layers will gradually bleach over time; I forget the order, but you could look it up if you have yellow skinned hens
- Squatting- laying birds will squat as if for a rooster when you approach them correctly/ hover over them. Not all layers do this for people, but it's very common and layer- specific
- Singing 'egg songs'- Most hens only do this if they are laying, and it's a good general indicator
- Feather quality- hens who are laying regularly should look like it. Unless if they just molted, hens shouldn't have entirely sleek, glossy feathers, as most of their protein is going toward egg production. Also, molting hens never lay.
Hopefully some of these help. You could also try trap nests, which would keep hens in the nest after they laid. Good luck!