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Spark plug tester really says little that is useful. Same could be learned by connecting to spark plugs mounted atop the engine. Crank the engine (someone else inside) while observing plug spark. Each spark will be strong and obvious.
Ceramic around the center prong should not have anything caked on it. Problem is if carbon has formed that would short the center prong to plug's metal - a path that would not create a spark. Carbon means the engine is eating oil.
More problematic would be a spark plug of the wrong heat range. You cannot test for that. Strange, but when I tried various heat ranges, it did result in poorer operation. In the days when normal was for an engine to leak oil, we would use a plug of one heat range higher. This burned off oil but also caused the engine to run poorer.
I have seen where another manufacturer's equivalent plug did cause some lesser performance. IOW I had trouble with an AC Delco equivalent. Ended up replacing the NKGs.
Since those days, we no longer replaced spark plugs. Electronic ignition and better machined engines eliminated spark plug replacement.
Since plugs are out, then just replace them with new ones. Plugs are cheap. Nothing can properly test a plug and what is important - its heat range.
Two electrodes or one makes little different. At one point, scam manufacturers were one hyping a superior design. In reality, the plug only creates a spark across one path - not matter how many other paths might exist. However using only various plugs recommended by the manufacturer has (in a rare case) proven important.
Be very careful about restoring plugs with proper torque. Over torquing is why plugs break off, strip threads, or seize in an aluminum block engine.
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