How to properly check transmission fluid and how often do I need to change it?

I just bought a used Mazda truck. It currently has 85,000 miles. The other day I checked the transmission fluid and it appeared way over the full line on the dipstick. I was wondering if the fluid has to be checked while the engine is running or at least warm. I checked the fluid in the morning on a cold engine that had not been turned on since the night before. Also, how often do I need to change the fluid and filter? Thanks.