Perhaps the ideal way to check ATF level is to really get the transmission fluid hot, by driving at least 15-20 miles, then check with the engine running. My tech puts the dipstick on his cheek to see if it's hot. If it's so hot he can't stand to leave it on his face, the transmission is hot enough for an accurate check. You would then use the upper mark on the dipstick to check, and you need to look at both sides of the dipstick. Sometimes it's hard to actually see the fluid, especially if it's clean, so I sometimes have to check it 3-4 times to see if I'm really seeing the true level (and my tech does the same). You also want to leave the dipstick in for a couple seconds before you pull it out for a reading.
You'd think this was a simple, trivial thing, but it's not. . .