Originally Posted by Honeysuga
The average sole is 3/8in thick. Most will not vary too much unless removed by a farrier.
The average *healthy* soles may be, but it's very common for horse's soles to be too thin to provide enough protection, far thinner than 3/8" and that farriers paring too much sole is but *one* reason for thin soles, not the major one.
OP, there are a number of 'landmarks' on the outside of the hoof capsule that can give you a fair idea of what lies beneath. As mentioned, the 'collateral grooves' - that is, the junction of sole & frog - are one. How deep or shallow these are, particularly at the point of the frog, will give you an idea. How concaved or otherwise the sole is, whether that concavity continues right out to the walls or whether there is a flat section around the outside. How much flaring/stretching there is, and whether the sole yields to pressure.... I've felt some soles that yielded to pinky pressure!
. Any one of these 'signs' alone may not mean much though, as concavity etc is different for different animals, and the only precise method does seem to be taking xrays, but considering all those 'landmarks' will give you a good idea.