How can you determine the exact age of your puppy?

hello all, im new to the forum And have a quick question. my name is Drew by the way for all of you single women lol. seriously, i got my new pup last month from a young lady and he was about the size of NFL football and his ears were still pointed up and not drooping yet. She told me the pups were 2months old. at first i believed her but then i started to compare my pup to other APBT pups which were the same age and noticed a big difference in size and thickness. (I know all dogs vary in size and weight) But the parents of him were a nice size. The father was a year old and was the height of a Doberman and the body of a APBT. The mother was a little bit bigger the standard size of APBT. So now im wonderingif my pup is actually 3 months old or maybe younger. Is there any way of finding out the exact age of ur dog or an great estimate?

P.S.
He was not the runt of the litter either he stood out the most.