There exists a device that determines if it is lead or not, but few wardens have them. They are too expensive, according to a warden friend of mine.

It comes down to whether or not the warden believes you. I take a picture of my reloading station with a bottle of Bismuth brand shot sitting there, to show them. They also may ask you for a shell to dismantle.

May God bless America and those who defend her.