The only way to check plugs is to remove them and inspect. As far as a missfire, the computer should pick up on it and the check engine light should come on. If that has happened, have the computer scanned to determine which cylender is causing the miss.
As far as the fuel, it shouldn't have hurt anything.
Let me know if you have other questions.
Joe
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Monday, July 26th, 2010 AT 11:55 PM