I check it cold because the "dipstick full mark" represents the "oil capacity" and I can't measure that when there's engine oil which hasn't drained down into the crankcase from the valve train on an engine that was just shut off.

You can check it on a hot engine but if you need to add oil, only fill it to the 1/2 way point between the FULL and ADD marks on the dipstick

What does your owner’s manual say?

Cold, but the oil doesn't expand so much that it will hurt anything if you overfill a little bit if you're doing an oil change. just put it to the top line on your dipstick either way and it will be ok. And if you're not sure how much it was filled last oil change and you wanna check, as long as it's in-between the two lines hot or cold then it prolly isn't burning any oil and I wouldn't worry. But ultimately a cold engine will give the most accurate reading.