Complete the application on the Certificate of Residency website. 2. Generally, you are considered a New York State resident for income tax purposes if you are domiciled in the state.Proof with a P0 Box address in not acceptable (except for NYS income tax returns). Gov (search: definitions) to determine your residency for income tax purposes. Proof with a PO Box address in not acceptable. Generally, employees have up to 90 days to establish City residence if they are residing outside of the City at the time of appointment.