Not sure if your tester will work, but I would definitely check it for codes.
If no codes are found, I start the car with the coil /spark plug covers removed and unplug each electrical connector at the coils one at a time, and seeing if the idle changes, then plugging them back in one at a time.
If the idle does not change at a certain coil, then that's the one misfiring.
I then try a new spark plug, then supressor end, then coil.

Usually you will get a check engine light on with the misfire.
Also your mass airflow sensor could be acting up, or your fuel filter could be clogged, or fuel injector, but the usual culprite is the supressor end or a coil that you will find with idle and low power rough running along with a ticking noise.