any way of guessing the sex at an early age?
There *is* a process known as vent sexing that does involve the examination of the internal structures of a chick's vent, but that is not exactly how it's done. The vent of a day old chick is carefully "opened", not by blowing on it, and the structures are examined. This is an illustration that might help show why it is not as simple as he understands it to be:
There are ways to determine gender at various ages - many of which depend on the specific breed that is being sexed.