With a magic 8 ball...

j/k

Warm up the transmission, leave it in Park for a minute or two (engine running), pull out the dipstick, wipe it clean with a lint-free cloth, put it back in, pull it out, and look for the lowest point where the fluid runs the full width of the dipstick. Should be between the min and max notches on the stick (or, if the engine is cold and running, which may be preferable from a skin-burn-due-to-dipstick-or-exhaust-manifold perpective, max should be 10mm below the "hot" min line).

I think the stick sits in the drain pan at an angle, which makes finding the level hard. I've never been very good at it. I just make sure the transmission isn't leaking a bunch, and that the level isn't too much one way or another.