There is no way for the pension payer to know where a person resides, just by their mailing address. Mom could be in a nursing home in Massachusetts but getting her mail at son’s home in Vermont. But the federal law is clear that states can’t tax pensions paid to nonresidents.

4 U.S. Code § 114 - Limitation on State income taxation of certain pension income
(a) No State may impose an income tax on any retirement income of an individual who is not a resident or domiciliary of such State (as determined under the laws of such State).