That's a good question. I haven't seen a utility that tells you that info. Everything I've seen is more just "whenever you're running at XXX FSB, your strap is XXXX."
If anyone has seen a utility for this, post a link .
get mbench and look at the latency. try 400 and 401 or something like that and you should be able to tell. for instance, i'm in the 40's-50's when at 400fsb. at 401, it goes right up to 60's-70's.