Dent depth is graded shallow, medium, deep or extreme. In international terms that is under about 0.91 mm, 0.91 to 1.25 mm, 1.26 to 1.40 mm, and over 1.40 mm. The numbers are tiny, which is why depth is harder to learn than size.
One practical trick: lay a credit card or business card across the dent under good light and look at the gap and the reflection. Over time, the repair itself teaches you most, because you feel how much metal has to move.
Curved panels make depth harder to read. When you are learning, start on shallow dents and work your way up.
Go deeper: this tip comes from the PDR eBook. See what is inside the eBook.
Prefer hands-on learning? See training dates. More PDR tips.