Regrettably, there's no backdoor way to verify this.
Talk to the seller, and see if he can go back to the estate to ask for any associated paperwork - now that he's sold it, he may be helpful.
As an aside - when looking at engraved Colts and those with possible Custom Shop work - before you really think about buying, get some proof up front, because factory engraving adds a different level of value to the piece, as does any of the Custom Shop work.
Scouts Out!