General rule of thumb: residency is living in a state 6 months and 1 day. Other items that the service looks at; where do you have a drivers license, where are you a member of a church. These are indicators of residency.
That being said; many states are looking to maximize revenue by creating rules that require filing a tax return for certain events. An example is you are a resident of one state but own property in another; you sell the property in the other state creating income. Many states will want to tax revenue from capital transaction occurring in that state.