[Vwdiesel] questions about re-using stretched bolts

[Area31 Research Facility]

Can anyone answer these two questions about bolt metalurgy?

I was wondering how a mechanic might know if the head or  rod bolts he has
have been used and stretched before when using them in an engine
build/re-build.

I'd also like to know if one has known used bolts,  can heat treasting make
them as good as new again to be re-stretched?  I'm thinking of the process
involving heating in an oven to the correct color and then quenching in oil
or air and then reheating to temper.

Thanx for any discussion.

Rob