Is there an easy way in C# to validate an html file (.htm) ?
You could try the article C# Validate XHTML on Dot Net Perls.
If you are not tied to doing this in C#, then usually the best way to validate HTML is using the W3C validation service.
You could of course, write some kind of test harness in C# which pushes all your HTML through the W3C validator, if it makes sense in your case.