It doesn't matter whether he signed the alleged statement or not, unless that "statement" was in fact a valid and properly executed/witnessed will. Only a will controls what happens to a person's assets after death; no other document, statement, etc. has an effect. So if there is a will and it directs what happens to this check, follow the will's instructions; if there is not, then this asset, along with your father's other assets, will be distributed according to the rules for intestate succession (who gets what when there is no will).