FWIW, the reason you don't encounter a "shell checker" in the field too often is a combination of their cost, lack of 100% reliability and more importantly the most accurate machines use fairly powerful rare earth magnets that present a health hazard through exposure to them. If you want to have some fun with the warden tell him you know what it is and how it works and let him know you'd like to step back about ten paces before he turns it on.
On more serious note although I agree with Steve it has hard to ignore the trampling of freedom by that law for making it an offense to simply have lead shot on one's person when waterfowl hunting. This presumption of guilt prevents one from carrying a couple of lead slugs (they are not shot but many agencies will ticket a duck hunter for having them) when deer and waterfowl seasons overlap. Similiarly when crow hunting there are many times during the early goose season to shoot a goose. Geese usually give plenty of warning of their approach, enough time to switch to a heavy load of big non tox shot but if you shoot at the goose you have all the lead crow loads on your person which breaks the law. IMO the law is flawed, it should be what shell was used to take a bird not what you have in your pocket. It assumes one is a criminal before you pull the trigger and that is wrong. The same approach used elsewhere would make one a drunk driver for having a six pack in your truck or woman guilty of prostitution because she has the plumbing.