where to measure to determine if trailer is level

This is my first trailer, I understand the importance of leveling the trailer. We did buy some of the lego blocks for under the wheels. I am wondering where do I put the level to determine if the trailer is level? Do I put it on the frame or maybe the side wall of the trailer, or perhaps the floor of the trailer? Once I get it level, I plan on placing a couple of the stick on levels on the front corner of the trailer.

Thank you