I knew a lot about babies, but it is always different having your own because you are with them all day and night. I was wondering too. It does come to you.
Are you using real nappies or disposables? Disposables feel full when dirty, but real nappies can be harder to feel, however, when Katie's are full it often feels warm. She also sometimes cries when she needs a change, but like people have said, check them at each feed (or at least every three or so hours - if baby is having a growth spurt it will have frequent feeds). Also before bedtime, and if baby wakes during the night, but you will get to know your baby. It is strange how that happens.
Another thing is that breastfed babes don't really smell that much (at least Katie doesn't).
Also a good time to check the nappy is just before leaving the house.