I've typed this out many times in the past few days, so decided to put it in an article so I can link it in responses.

Got pullets newly laying and not sure who is laying and who is not?
This also applies to older birds.
Under the tail will tell the true tale, so part those fluffy feathers and take a look!
Yeah yeah, might sound gross, butt (haha) it's a good thing to learn how to do.
Might not tell you who is laying what, gotta catch 'em in the 'hot egged' act for that, but if you start early and every time you find a tiny new egg, do the butt check to see who the new layer is, and it might tell you who is laying what.

Vent Appearance:
Dry, tight, and smaller - usually not laying.
Moist, wide, and larger - usually laying.

Pelvic Points:
Feel for the 2 bony points(pelvic bones F-F) on either side of the vent:
Less than 2 fingertip widths apart usually means not laying.
More than 2 fingertip widths apart usually means laying.
(Spacing is relative to chickens' size and humans' finger size.)
(Spacing also relative with bantams, compare laying to not laying birds.)
With practice and experience, it will get easier.


Here's another illustration that might be helpful (the pelvic points are labeled 'pubic'),
tho I have no experience with the distance between vent and breastbone:
Chicken Pubic and Keel bones.jpg

Other new layers notes:
New layers can act quite goofy - they don't know what they are doing at first and can be confused and anxious, it can take up to a month or so before they get it all figured out. Putting some fake eggs or golf balls in the nest might help show them where to lay. They may scratch around in the nests for weeks before laying, spreading the bedding everywhere. They will scratch around a bit less in the nest as they get used to the routine. Meanwhile, eggs everywhere, some of them can be rather funky looking, soft or thin-shelled, huge double-yolked eggs.

Signs of onset of lay---I've found the pelvic points(see above) to be the most accurate.
BUTT(haha!) messing of the nests is the surest sign that an egg is imminent.

If you touch their back they will hunker down on the ground, then shake their tail feathers when they get back up.
Tho not all birds will do this, especially if there's a cockbird in the flock.
This shows they are sexually mature and egg-laying is close at hand.

Combs and Wattles:
Plump, shiny red - usually means laying.
Shriveled, dryish looking, and pale - usually means not laying.
Tho I have found that the combs and wattles can look full and red one minute then pale back out the next due to exertion or excitement can drive ya nuts when waiting for a pullet to lay!