How do I go about finding my max?

Also form and technique criticism is welcome.

Excuse my hair, it does whatever it wants to do. Anyways, I confused myself a beginner with a little over a year in the gym. I started putting deadlifts in my routine 3 months ago. This video was last week and the sets were

Warmup: 65lbs 1x12 155 1x8 165 1x8 175 2x8