Checking spark strength on a spark plug is important to make sure that the plug is properly arcing. Insufficient spark will hamper ignition in an engine and make it run badly. Eventually, combustion will not occur and the spark plug will foul from too much fuel and no burn-off. However, the evaluation process is fairly easy and can be performed with simple tools.
Carefully pull off the ignition cap wiring from the spark plug in the engine being worked on. Insert a socket wrench and appropriate size socket and loosen the plug. Pull the plug out of the engine. Carefully wipe the plug insert cavity with a shop rag, making sure nothing falls inside the engine.
Take the old spark plug and insert a plug gap tool between the arc of the plug and it's point. Measure the gap. Refer to your vehicle repair manual for the proper gap required. Compare your result with the manual's data. Use the tool to bend the arc until the gap is the proper distance.
Reinsert the old plug into the engine after cleaning it up with a shop rag. Twist it in by hand to avoid damaging the engine threads. Tighten carefully with the socket wrench until it is secure. Do not over-tighten or you will strip the plug threads.
Attach a spark plug tester tool to the top of the plug. Do not attach the wiring yet. Crank the engine flywheel so that the engine turns over. Monitor the tool as it lights up to confirm the plug still produces a proper spark at the appropriate cycle timing (when the piston compresses inside the engine against the internal spark end).
Take the plug out with the socket wrench if it does not light up the tool. Replace the plug with a new one after making sure it is gapped properly on the tip. Tighten it carefully with the socket wrench. Re-do the spark plug tool test in Step 4. Re-attach the plug engine wiring and cap when satisfied. Start the engine and run it for a while to confirm the tested plug is working properly.
If you don't have a vehicle repair manual for your engine, you easily find the spark plug gap measurements for your engine on the Internet with a few word searches.
Make sure the engine is off when working on it and removing the spark plug. Never force the spark plug to tighten. If it doesn't seem to fit right, take it out and reinsert it. You've likely misaligned the threads, and forcing the plug further will damage them, which can result in a costly repair.