Honestly, you don't need to. I guess it's something you "learn to feel" over time, but you don't need to put any significant amount of force onto a spark plug. Just go until it's tight and then maybe a bit more, especially if there is a washer or crush washer. Keep in mind we're often talking about aluminum heads with aluminum threads that can strip out very easily and tapping a stripped out spark plug hole really *****. I once had a stripped out bolt on an idler pulley break off the front of the block and grenade under the hood damaging the timing cover, coolant reservoir, shredding the belt and other components.
I'm not going to disagree, I know most manufactures these days like NGK don't recommend you gap their plugs at all.
But it's still a good idea to at least check. Beyond any obvious damage (like a dropped plug), I can't eyeball the difference between say .030" and .040" but it could mean the difference in how the spark plug functions/performs. I also know most plugs these days have cardboard sleeves over the end to help prevent damage even if the box is dropped or something heavy is put on top of it.
I don't see how gently sliding the plug over or onto a gap tool until it's tight would cause any damage. I say just don't be a stupid 200lb gorilla about it or otherwise try to force it. But that's just me.
EDIT: Aside from the older styles that use wire or looks like a coin, or even a feeler gauge, I like this style...
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