You really can't, when I buy files or rasps I heat treat them and quench them. I put them in a vice and hit them with a hammer if they break they will make a good knife. If the bend, they go in the trash. Some do a spark test but I'd rather be safe than sorry.
I normally just do the tail of the rasps and files, that way I won't take too much off the stock I'm going to use. I heat them in a gas forge to test them.
Last edited by bjankowski; 05/22/21 09:09 PM.