its hard to explain i mean if its a real good one u prolly wont be able to tell, but usually u can tell like if someone gathered their breath again (but there was no time to gather) and the timing on the line is still right. uh i mean sometimes rappers do it on purpose, also u can tell theres a punch in whenever they have those effects and stuff, like nicki minaj on monster so thats a key give away. Usually depends song to song tho, i guess just if the rappers seems like they expended mad energy on the verse then they're getting towards the end and theres like no way they can complete it in ur mind but they happen to.
most of your favorite rappers punch in tho, the only one i know that doesn't punch in at all is jay-z thats why his shit sounds so much more natural. If a dude is going in on that mic, but you never hear him take in a breath really somewhere hes prolly punching. Rappers that use alot of effects in their verses, those are the ones that usually punch in alot.