I know the obvious such as massive rpm's with no movement and the noticable slipping feeling...

but if I dont have any of that what are some other ways to tell?

If I was to have my tranny checked out if I pull it out and take it somewhere, what would they look at? could they tell just from dropping the pan or would they have to go into the tranny to find something wrong with it?

If it doesnt need a rebuild I would hate to pay for it just for them to make some money off me. know what I mean!?