My dad is a luthier of 15 years and he's developed his method of working on fretboards for quite awhile. It's common to use a flat edge and feeler gauges, this is still useful, but he's developed a technique of leveling under tension that has been very successful. He wanted to collect his knowledge and pass it on so we made a video demonstration of this process and wrote a guide. Check it out: How to dress and polish frets