Rusty to beauty -- but how to tell if it can be made shave ready?

[sg.]

Following images show a rusted straight transformed into a shining beauty.

1. But features are needed in the result to indicate whether it can be made shave ready?
   
2. And could these features have been inferred before the start of the restoration?

 

[bluesman 7]

It's often recommended to at least set a bevel before restoring to see if there is anything worth restoring.

 

[Alum of Potash]

One way would be to look for congealed, blackened rust especially along the edge. The colloquial term is "devil's spit," and it can often be felt with the finger as well. Devil's spit has a tendency to penetrate the metal, not just lie on the surface; and at the very apex, or the edge, it can penetrate throughout, turning up as pinholes and chipping in setting the bevel. I think it has to do with lingering soap scum being left on the blade for an extended period of time.

The images as shown seem to be of red surface rust, although images can always be misleading.

That said, a small minority of beginners like a devil's-spit edge as it gives them an authentic reason to use a full honing progression on the same razor over and over again, as the shaving edge quickly deteriorates, which provides more practice on the same blade without damaging others.