1). On the trailer, adjust the height of the bow so that your keel is exactly level.
2.) Take a level and hold it vertically up against the transom at the top of the splash well and hold it perfectly plumb.
3.) Have your helper take a sliding bevel and match it to the transom and the level.
4.) Transfer the angle to a piece of paper and measure with a protractor.
There are probably many ways to do it, but this is what came to mind. Good luck!