I would put the valve cover on a piece of glass and look at how it sits...that is what I do with it to start, that will tell you if it is flat and how the sealing surfaces look. I would start there, if it is in question, use some aluminum oxide paper to sand it down flat...using the glass to hold it flat.

1941 Taylorcraft
1968 Charger
1994 Wrangler
1998 Wrangler
2008 Kia Rio
2017 Jetta

I didn't do 4 years and 9 months of Graduate School to be called Mister!