You don't need to worry about checking the transmission fluid, it won't last long enough to get too worn before you need to replace it anyway.
Well perhaps this is a touch of bitterness that mine failed at just over 60k miles and had to be rebuilt.
Slightly more serious - You can't check it, but there is a transmission fluid change schedule so it be replaced when needed. You could always have the fluid changed proactively if you are worried about it.
Ram Promaster 3500 transmission dipstick
Linear Mode
