Umm, you can't. Well you can but you really won't get much usefull info.
You can check for continuity from the outside hv connection to the center inside contact. Sometimes that will open. But for "resistor" plugs (the most common kind) you will read a resistance even if good. You can do a resistance check from the outside hv connection to the hex base, that should be open, but even good used plugs have some carbon deposits that may show some leakage.
The best way is to substitute a new or known good used plug and see if that helps. Or better yet, cough up a few bucks for a new set once a year or so.
And every 3 or 4 years a new set of wires isn't a bad idea either.
My shade tree mechanic test of a spark plug is to have it connected to a plug wire and laying on a metal motor part such as a head. Briefly start the motor and observe the spark for regular frequency, "fatness", and bright color.
There are some obvious safety risks, so think carefully if you want to stay completely safe or to use this method.
Plug wires are generally of terrible quality compared to decades ago. It is now very likely that the first time wires are removed, one or more will become internally cracked no matter how carefully it is done. If you test a wire for resistance and find infinite resistance, it is, of course, broken. In many cases you cannot get the meter probes onto the ends of a wire without removing the wire. Catch 22. Needless to say, do this test with the motor DEAD.
less hate, more science
thanks. I was looking for a way to test the plug without taking it out, as I have a sideways engine (as many cars have now) and the 3 plugs in the back are hard as fuck to get at. I am not even sure I can get them out without removing the engine.