How to inspect an anti backfire valve?

[Toronado3800]

Was just peeking around mine and noticed a little oil. Even sticking my phone in there I can't get a decent pic w/o loosening up the power steering pump or pulling the intake.

Is there anything visible to see?  What's the method of testing it?  What are the symptoms of one gone bad?

I'm chasing a backfire and slight miss in another thread.  Maybe this should be its own or maybe merged.  I'll let the forum gods decide

[RCA1186]

I think a plenum smoke test would reveal any leaks related to the valve. I think you would hear it if it was leaking though.