It is hard to tell. Sometimes they run well after you swear they are fouled, sometimes they look almost new and won't fire. The easiest way I found to check for a fouled plug is to put in a new one. If it runs OK, toss the old. If it was running badly before it died, it may take a while to run clean, even with a new plug. Also every engine is different. I was lucky to go 800 miles with my old Xpress. My Xplorer went over 2000. Then I put in a new plug & kept the old in the toolbox as a spare for the trail.