Just the VF x current

but, who do you figure out how much power the package can safely dissipate?

So take any Diode and the data sheet says it can run an absolute max of 20A

ok great, we all know that is under very specific conditions.

Let’s say I want to run 10A through it and we know the Vf is .5

ok so 5 watts

How can I prove that the part is under its thermal limit? Is that also listed in the data sheet?