I sometimes create a sequence just for this purpose naming it "scratch", (now part of my templates). I will load two copies of a clip into two tracks, one on top of the other. Apply the desired FX to each, and then turn the top track visibility on/off as I watch the monitor, (no need to move on the timeline as you'll get frame accurate comparison). Then either copy the selected FX to the original clip in the edit, or save a custom preset to apply to similar clips going forward.
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