What kind of helicopter is it? Something you want to keep in mind is wind. I like to center mine on a very calm day. If the wind is blowing to your right, the heli is going to want to roll that way. Same if the wind is blowing in your face, the heli is going to want to roll, flip towards you. And it doesnt take much. You may think it is off center, but really its not.
Now thats not to say that your heli isn't off center. A good way to check is have the blades straight out from front to back (i always take my blades off so there is no mistakes) with the paddles going left to right. Grab the heli by the flybar with your fingers as close to the head as you can without touching the seesaw or head. Pick it up and the heli should stay level.