Double taxed income: How can I know I'm double-taxed?

I'm a part-year PA resident, and I worked at NY during I live in PA. Moreover, when I file PA state tax return after finishing the NY state tax return, it looks I need to fill the form titled in "Any Double-Taxed Income from New York?"

 

In this case, how can I know PA tax on the income from NY? In other words, how can I fill the "Gross Compensation" row and "tax Due to New York on income above" row in the form I mentioned above?

 

Thanks!