Transmission fluid needs to be checked when the engine is idling and the transmission fluid has warmed up.
Most transmissions will require the engine to be either in park or neutral and the engine idling before checking the transmission fluid.
You should drive the car or vehicle around for a few minutes to allow the transmission fluid to warm up and pump throughout the transmission.
Then either put it in park or neutral and leave the engine idling and then pull the transmission fluid dipstick out and wipe the dipstick down.
Then insert the dipstick back into the transmission and then pull it out again to check it.
Then you'll get an accurate reading.
If you need to add some transmission fluid you should add a little at a time til you get the transmission fluid back to the proper level.
Make sure to use the correct transmission fluid as well otherwise it will destroy your transmission.