Go to most fitness shops, they can do it for you free-of-charge. But if you wanna do it yourself, can try getting one of those body fat calipers.

A weight that's not too light, or too heavy for you to finish the necessary reps per set. It depends on the individual.

Yeah, its popping up everywhere I go apparently. But its not cheap man, $39 = RM100+ for a piece of foam. Not worth it in my honest opinion. Can always just get a DIY solution to make the bar bigger if that's what you want.