It is always the actual location of the station. They should indicate somewhere on their QSL card what their location was for the contact.
For our informal TVARC WAS, no proof is required. If you need formal proof, perhaps the POTA database of operations might corroborate their location during your contact.
The question for all you DX’s. If you were working on WAS and you work a POTA station whose home is in one state, but the park is in another state. Their QSL card says that they were on PO TA, but does not mention the state that they were actually operating
in. Which state you assign that QSO to?
Mike
K6UMO