The trans level can be checked with the aftermarket dip stick, or :
when I change my fluid, I do it at room temp and suck it out with a vacuum sucker.
I mark the high level of the fluid on the sucker tank, remove the pan and change the filter ( and conductor plate at this point )
put the pan back on and fill the vacuum device with the same amount of new fluid, then pour into transmission.
bada bing - bada boom