Well first you want to desperately look for any paperwork.
If it was ever registered, you are probably in good shape. But you definitely don't want to rely on ATF records to verify that it is registered.
If it isn't registered, you are pretty screwed. Even if it was brought back prior to 1968 you are gonna have a hard time proving it unless that fact is somehow reliably documented. And if that is the case, you "might" be able to register it under a "war trophy" amnesty.
But if you have none of the above, for all ATF knows, it was brought in illegally last week. Under that scenario your options are basically donate, surrender or destroy.
You can donate it so a museum or LE agency.
You can surrender it to ATF (who may very well destroy it).
Or you can destroy the receiver and keep or sell the resulting part kit.
Forget about trying to sell it to a FFL/SOT, you'll probably end up getting screwed out of it. First they know that your mere possession of it is technically illegal. And even if they could get a LE demo letter to legally acquire the post sample machine gun from you, you can't be in legal possession while you wait for the paperwork to clear.
Which means most FFL/SOTs will try and take possession of it without any form of compensation "as a favor" to you.
My advice is if you are on good terms with a local Sheriff, consider offering to donate it to his agency with the request that you can get regular trigger time on it. And while it is in his custody, you can research ATF to find out if it is on the registry according to their records.