The main thing you can do to check the transmission is to inspect the fluid color and smell. If it is brown and burnt smelling, then the trans was probably not serviced on a regular basis. If it has an overdrive lockout on the shifter it's probably an e4od, if its on the dash probably an aod. Both transmissions are fairly reliable and will last quite a while before failure. Ask if the previous owner did any towing,and when the fluid was last changed. check for external trans coolers thats a good indicator of regular towing. Take it for a drive and before leaving shift from park to reverse and notice how long the transmission takes to pull against the brake, it should be instantaneous. Reverse takes the most fluid pressure to engage the clutch pack, so this is a good indication of the condition of the tranny pump. When you take off and under steady light acceleration, feel the tranny shift. If its not concise but understated then you got a problem. What I mean is, you feel the engine RPM rise or see it on a tach and you should feel the tranny light but definitely shift no hesitation no hard shifts either. do this several times to warm up the tranny, try to feel if the shifts are happening at the same speed each time consistently and with the same feel. If the tranny feels like its searching for a gear or cant figure out which gear its in, walk away, it'll get expensive. Tell the seller you want to have a mechanic do a test drive and a pressure test and pull the codes, see if any are transmission related. Have the dealer do an inspection on it before or after purchase, its cheap insurance. Good Luck