Parked on a hill - Automatic transmission difficult to shift

[DanSanDiego]

Today I parked on a somewhat steep hill (driveway) because I am trying to diagnose a small leak and wanted to see if I can clear any fluids from on top of the stiffening plate and splash guard.

Anyway, when I started the car and tried to shift into gear it wouldn't shift. I tried again and it wouldn't go. Then I tried a third time and it went into reverse with some pressure.

Is that normal?
Could it mean that my transmission fluid is low?
(Is there a way to check the level of the transmission fluid without opening the fill plug?)