How can I tell if they have started laying yet?

I have 3 older hens and 2 younger hens. The 3 older lay eggs. I was getting 1 egg a day then the next day 2 eggs and so on every other day 1 egg and every other day 2 eggs. I knew who was laying what... but for the last 4 days I have been getting 2 eggs EVERY day... and well that would mean one of the 3 girls is laying more frequent then she should be OR 1 of the younger hens has started laying eggs...  I did a butt check... LOL I looked at the 2 younger hens vent to see if it looked larger because I know once they have started laying eggs their vent gets a bit bigger... well the larger of the 2 younger's looked a bit larger than the smaller of the 2 younger's but then again it could just be because she is a larger hen... IDK is there any for sure way to know other that seeing the egg come out... The one I think it might be is almost 4 months old... so I don't even think she is old enough yet..

if a hen was laying more than 1 egg a day would she still be consistently laying more than 1 egg a day? this started sunday... so 4/5 days later?

That's is what I was thinking. so... it means we have 4 egg layers more than likely... hmmm she is between 15 to 18 weeks old... I am guessing closer to 18 than 15 judging by her size compared to my other hens. She is the dark barred rock mix in the below pictures... in the first picture she is the one closes to the coop on the right.

she is the black and white bared mix -------^

I did have 1 or 2 eggs that were a bit smaller then the rest but not a bunch bunch smaller like you would expect a trainer egg to be.

YAY how exciting. I am so thrilled. LOL yeah you would figure they would shrink in size any huh? so yay! I am excited. now I just need the other to start plopping out eggs and then I have all 5 laying. YAY YAY YAY!