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[updated:LAST EDITED ON 03-Jan-03 AT 11:35 PM (EST)]Checking isn't really possible with them installed - you need a special test rig, but it's kinda moot. If you think it's bad enough to pull one, just put a new one back in.
I just run some decent FI cleaner after every other oil change (~5000mi), and I vary which brand & type I use. Sometimes the $3/bottle stuff and sometimes the $10. Even though mine were so bad when I bought the engine that I had to short them for a minute or so to get them hot enough to open, they've run perfectly for the past 4 years.
http://www.webphotos.com/list_photos.asp?mi=3&smi=1&a=13217