Each state with an income tax has laws that require non-residents to pay income tax on work that is physically carried out within its borders. So, in order to be in compliance with these laws, you must keep track of the income you earn in those non-resident states. You must file a non-resident tax return in each state in which your income exceeds that state's filing threshold for non-residents. The only exception would be if your resident state has a reciprocity agreement with the non-resident state.
When two states have a reciprocity agreement with each other, a taxpayer who lives in one state but works in the other only has to file taxes in his or her home state. Kentucky, for example, has reciprocity agreements with seven other states. Unfortunately for you, Delaware does not have a reciprocity agreement with any other state.
You can find a given state's filing threshold for non-residents on its tax website. Be aware that the filing threshold for some states is based on the taxpayer's total earnings from all sources, not just on the earnings from the particular state.
Some large companies now have payroll systems that allow them to track and even to pay withholding taxes for their employees' non-resident state earnings, but this is still uncommon. In most cases, it's up to the employee to keep track.
Federal law prohibits two states from charging income tax on the same dollars, so your home state of DE will allow you to claim a credit on its tax return for the taxes you pay to non-resident states. This in effect prevents double taxation.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
