Good title; bad title. How do you verify that someone owns copyright? You can start by searching Copyright Office records, or you can hire a search company to research the copyright history. Search results, alas, are not always conclusive. The heirs may not yet have registered ownership in their name, may not have filed documentation evidencing the transfer, or they may have filed documents that have not yet been recorded. There is also the possibility that the records reflect that the heirs own copyright ... but don't reflect the fact that they have since transferred rights. Finally, the heirs may have provided incorrect information to the Copyright Office. A registration doesn't verify that the person owns the copyright; it creates a presumption that the person owns it.
Assurances. So, a wise approach may be to research on your own and to also seek assurances in your option agreement. You can include warranties -- guarantees as to representations of fact -- and indemnities -- promises to pay for all damages and costs if a third-party sues over the ownership issue. At the absolute minimum your option agreement must include a warranty from the heirs that they have the power and authority to enter into the option agreement. You can also ask that the heirs provide you with documentation from the Copyright Office evidencing ownership (that may require them to file documents) and you may also seek other proof of the transfer -- a copy of the will, an opinion letter from the attorney, or documentation from the probate court.
Wild card department. Though it's probably not an issue, it may matter whether the work falls under the 1909 Act or the 1976 Act (effective January 1, 1978). This could affect the rights of the heirs to terminate previously existing transfers of ownership. It's confusing stuff and a lawyer's assistance may be required.