[Fot] Verifying if a crank surface is nitrided

[Scypher at USA.com]

I have a number of spare spit 1500 cranks that were clearly nitrided  
at some point in their life, but are at the 020 030 and even 040 under  
stage of their life.

How do I determine there is enough of a nitrided layer left?  I need  
to decide about prepping a new crank and nitriding, or turning down an  
already prepped crank to the next size.

Can anyone verify if stock 1500 cranks came with any nitriding from  
the factory?

Thanks
-Scott