How will I be able to touch a man Penis while he's fallen asleep?

My elder sister's husband often pretend as if he was fallen asleep,and one night while I think he was sleeping I tried to pull off his pants and touch his penis and take the pic of his undie,but he was conscious,and I feel so shy making lame excuse like 'I tried to wipe the dust off your pants'.So how will I be able to know that he has really fallen asleep or concious,and how will they be when they're really fallen asleep?So that I would be able to measure the size of his dick.