my toddler avoids me at times when she's wet, so I have to check her (and have her scream NO at me as I am doing it) or leave it a while, till I know she needs changing (Ie it's sagging a fair bit). She's started coming to me to let me know she needs changing or wants potty, but it's not all the time and she HATES having her bum changed most of the time (mind u she hates poops so she'll let me know straight away to change her then, but wees, she runs away) so I agree with the sagging one. It's a bit of an exaggeration saying down to the knees, but nappies do sag when wet and toddlers have HUGE bladders!