If they are not sharp (no point or gouge in the bottom) then the spec only really limits you by tolerance.
And in pipe that would be one heck of a large dent.
I would start with micrometers and measure at the edges of the dent and see if the OD is still in tol.
If there is a sharp line, crease, point, or gouge in the bottom of the dent I would say that anything that you can feel is questionable.
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P.E. Metallurgy, Plymouth Tube