New Hampshire determines residency by where your driver's license is from. If you have an out of state license then you are a non-resident. If you do not have a driver's license then the Town Clerk can decide if you are a resident. Absent that, or in a disagreement, I assume that the courts could get involved.
Most states have laws that say if you establish a residence there and are gainfully employed for over 30 days you must switch your vehicle registration(s) and license to that state. There is a tremendous grey area.
I asked a DMV official in Florida if they thought every snowbird that went to Florida for the winter and got a job bagging groceries to supplement their income had changed their license and registration. They acknowledged that most would not but would be subject to arrest for unlicensed and unregistered if they were stopped. So, if this fits you, make sure you have the "right" answers ready if you get stopped. (No sir, no job, just passing through............)
