I wondered when the 6R questions would start.
I have an answer for you that you will like.
Well.... here goes: As compared to your 5R cousins, you are in luck. The 6R DOES have a dipstick. "WHERE?" you ask, since you you don't see it in the engine compartment anywhere. Well... it is on (under actually) the fill plug. There is a hexagonal "fill" plug on the passenger side of the tranny near the front. Pull this plug and you will find the "dipstick". With a warm transmission, and the engine running, tranny in Park - wheels chocked and or emergency brake on (safety first), remove this plug and then separate the dipstick from the hex plug (it nests in the plug - you will understand when you do this), wipe it clean and then replace it in the hole until it bottoms, remove to read the level. Add fluid as needed. Reassemble the plug and stick and reinstall.
VOILA, fluid check 6R style!