For me I have visibly dropped, I've got a bowling ball in my lap so my belly rests on my legs when I sit which also means I can't cross my legs anymore, lot of pressure, and I have to pee a lot more. But mainly I know because my OB did a check and her head was right there too.
Your healthcare practitioner will check engagement at your exams. He/she will basically put their fingers on the sides of baby's head and wiggle it to see how well it's 'wedged' in your pelvis. Once the head shows no movement in the pelvis, it's engaged.
Dropping is entirely different. You will be able to tell baby has dropped because you get a really severe waddle, and get that 'bowling ball' feeling. Your baby can drop into your pelvis without becoming engaged.
Re: Question
Your healthcare practitioner will check engagement at your exams. He/she will basically put their fingers on the sides of baby's head and wiggle it to see how well it's 'wedged' in your pelvis. Once the head shows no movement in the pelvis, it's engaged.
Dropping is entirely different. You will be able to tell baby has dropped because you get a really severe waddle, and get that 'bowling ball' feeling.
Your baby can drop into your pelvis without becoming engaged. 
Monster Truck (It's a GIRL!) is due 19/02/2015!