When planting a post in the ground, I use a "Post and Pipe Level". I got mine at Home Depot for $5. It's two sided (90 degrees) so you can see the plumb on two sides of a post at the same time. Before I found this, I used to plumb with a level, hold the post and move the level to 90 degree side and go back and forth. With the P&PL, don't have to do this.