These are a great way to tell if you have spark. well second to getting the Navi to hold it while you crank it over. but it sure beats using a screwdriver or pulling the plug out. sure its far cheaper to do that but think of the time wasted and these things are under $20. Just pull a lead and attach this spark tester inline and go... not fuel vapor issues or finding a good earth. you can even run the vehicle with it install for a short time without problems. it even fits onto a SR20de where the plugs are down deep in the head below the cams.
Inline Spark Tester