Haven't heard it mentioned, but you could use freeze spray and cool different (suspect) items down to narrow down the culprit.
I also have one of those Snap-on air powered choke testers that's pretty handy for this type of stuff. Don't ask me how it works but you put shop air in and it blows hot air out one end and cold air out the other with no moving parts.
OK, did a little digging. Snap-on no longer sells it but here is one:
http://www.exair.com/en-US/Primary%2...es%20Home.aspx