The best way to gauge sole depth is x-rays.
If a rapid assessment is necessary though, you can measure the concavity from the apex of the sole.
Another visual indicator is that the collateral grooves go all around the frog. Meaning that the frog seems to be nicely settled in a deep cushion of sole (provided this is not due to infection digging the collateral grooves).
▪️ This 1st one has very thin soles. The frog looks like it’s standing out, not really part of the rest.
▪️ The 2nd example has good sole thickness. The frog looks “hugged” by sole. [This horse lives in a surfaced Paddock Paradise, so has lots of stimulation.]