Check transmission

I have a Dodge Ram 1500 59 liter 360 4 x 4 I think
the shop that's going to rebuild the tranny is not
doing this proper diagnosis on the transmission
how would I check to see if anything else was
wrong with it cause the truck was running fine the
day before and I parked it overnight and came out
and of a sudden the transmission was slipping the
next day they want to charge us $1,500 to rebuild
his tranny and put it back in the truck so how
would I go about checking to see if something else
is wrong.