61 Pa. Code § 111.5
Credit will not be allowed under this section unless the taxpayer has proved to the satisfaction of the Department the amount of tax paid to the other state. A Form W-2 indicating income tax withheld for another state will not be sufficient to establish credit under this section. The mere fact that tax was withheld does not establish the fact that a taxpayer has been liable for the payment of tax to another state. Therefore, when claiming a credit for taxes paid to another state, a taxpayer shall file with his Pennsylvania tax return a copy of the tax return filed with the other state. The return will be considered as prima facie evidence of the amount of income tax, if any, paid to the other state.
61 Pa. Code § 111.5
The provisions of this §111.5 amended under section 354 of the Tax Reform Code of 1971 (72 P. S. § 7354).