How can I check to see if a used transmission is still good?

I'd certainly drop the pan, and make sure to change the filter. If the old fluid looks brown and/or smells burnt, you may be looking at a rebuild. It depends, how brown, how burnt. Look for debris, broken pieces, make sure the bands and anchors are in place and not broken. While the pan is off, you can adjust the low/reverse band. if you fell confident and have an inch/pound torque wrench. I'd also replace the front seal, now, because if you have to pull the trans again, just because it's leaking, you'll be kicking yourself. 10 mins and $10 vs at least several hours labor, your call.