The best way to test a spark plug in home conditions is to remove all plugs, put them in coils, put the spark plug (metal part, thread) on a engine metal part (earth), and turn a key, or better to ask someone to turn it. And see a beautiful blue spark in every plug. If there is any missing spark, put any other plug in that coil, so you can figure out if it is a spark plug or coil..
Sometimes spark plugs might give you a spark when removed, but won't give it in the engine. This is why the ideal way would be to test them under 12 bar pressure. But that would cost more than a new spark plug.
I have a 100% standard car, and I love high revs, so when I notice any misfire I just replace all the spark plugs. If you don't do it the disease goes into a coil and lead. After that one cylinder stops working, your Lambda sensor reads rich Air/Fuel ratio, because 1 cylinder does nothing, corrects the fuel injection, gives lean mixture, the rest of cylinders have higher load because one is missing and slowing them down, and they receive a deadly dose of fuel and air mix, and if you are not in limp mode or just drive slowly, you melt your pistons. This is the worst scenario in 1 faulty spark plug..