Will a dignostic test tell me if the transmission needs to be repaired? How long does the test take?

1999 Jeep Wrangler, straight 6, 4 liter. I put in a rotor, plugs, sparks, wires, air filter, and AC switch. (Needs) an ac compressor w/ clutch ok. Then I was driving, stopped at a stop sign, stepped on the gas to go. It spun. Had to put it in reverse and then drive and drove home. Changed the trans filter and trans solinod and fluid. It was not burnt ok. Doesn't slip while cold, slips when warmed up in drive, not 1st, 2nd, or reverse. Added a non-gear slipping agent. Still spins. Call for an appt for testing. The guy said it would take 3 hours (what) its a computer! I would actually like to know what does a dignostic test tell you and why 3 hours.I'm a female and I think these guys see money. Please help asap its my ride and I don't want to be ripped off. Low miles so I'm shock on the transmission issue.