Put a file on it. If annealed, the steel will be like butter to the file. If it is hard, hard as glass, it still has the hardness from when it was made. Color wears off but hardness does not. You can find a place on the receiver where it wont show and do it there. Your other option is to use an engraver tool which you may not own. Just try to cut a straight line. If the metal is hard it will float like over glass.
As to the cost to harden it if needed, after all the work you have invested, a few more bucks is academic. It is a hobby gun, a project, not a way to make money.
