Force them to figure it out or start impounding goods? Impound their goods if you can't, for any reason, gain access to inspect any part of their declared supply chain for occasional surprise inspections to verify it is as they say it is, until such time as they switch to recently-inspected suppliers or gain you access?
I don't really get how this is much harder or less practical for the state or states implementing the law than most laws governing corporate behavior, and particularly laws governing imports. Yes a law would have to be drafted, passed, and enforced. I don't think any part of that's particularly less reasonable or more unlikely than in the case of any similar law.
In short, you punish companies for either using conflict minerals or if you're unable to determine to a reasonable level of satisfaction that they are not using conflict minerals. Likely they'll want to make sure you can determine such, and will pressure their suppliers to make it happen, in the usual ways markets do things (suppliers that make it easy and do it right and develop a reputation for having few fine-inducing screw-ups can charge more, those that fail miserably at it may just go out of business and be replaced by others who aren't terrible at their job, pretty soon everything's a little more expensive but we have a decent idea of what comes from where)
