where to measure to determine if trailer is level

Lots of different ways mentioned...what I use is: I have two bubble stick on levels on the rear frame, each side. I have one on top of the rear bumper, two on the front, each side plus a small one near the tongue jack. I can tell when I back into a spot if it is fairly level and what side needs more or less. I also use a small level that I use inside. If I really want to get technical with it I have a level that is in an app on my Iphone. Check fridge door to see which direction it swings....