First you need to find the licensees/assignees. You would need to find the companies that licensed the patent from your grandfather. Check your grandfather's estate documents for evidence of those agreements or consult with your grandfather's attorney if that's possible. Alternatively, search patent records at the USPTO or Google patents for evidence of any assignment of the patent.
Now the hard part ... Even assuming you could track down companies that had licensed or were assigned the patent, you would still face the biggest hurdle: making a legal claim for compensation. What's in your way is the statute of limitations. Under most state laws, you have between 3-10 years (here's a fifty state chart) to file a claim based on a breach of contract. That date starts ticking the day when you should have become aware that the contract was breached. We don't know the details of your grandfather's death or his condition preceding death so it may be possible to make the creative legal argument that it was not possible for you to discover the breach of the agreement until now but the Dear Rich Staff sort of doubts that will fly. In any case, don't listen to us if you have any doubts; promptly take your paperwork to a patent attorney. All in all, we hate to see inventors go uncompensated (if that's the case with your grandfather) but we're not sure there's much you can do.
From our mailbag. Kim from Beverly Hills sent us news today that a case of patent litigation between two magicians has come to an end and the patent holder was victorious. Thanks Kim, we're glad the world is safe for the makers of levitating pens. (And yes, readers, the magic secrets are published for the world to see in the patent applications). Mostly, we're grateful to Kim for getting us to research the subject because it turned us on to this wonderful blog.