Wait, are these "real" meat birds i.e. Cornish Cross, White Rock, Slow Cornish, anything involving the words "white" or "cornish" or "rock" or "broiler" in their name? If so, you will have ZERO trouble telling them apart on arrival - they are built like little sumo wrestlers.
Or is it like a frypan special type deal of assorted large-fowl breed males? In which case, if your layers are also an assortment, indeed you may not be able to tell them apart.
If it is a mixed-dual-purpose-breeds situation, then you will pretty much grow them all together until it begins to become evident who's destined for what, and then segregate out the cockerels and feed them up a bit extra til they are eatin' size.
Good luck, have fun,
Pat