+1 the key is whether the rim seems to move back and forth as it spins, or whether it runs straight but is off center. The former is a wheel alignment problem, the latter is probably a mounting issue.

However, there's a third possibility if the wheel is straight and properly mounted. It's possible that the wheel is improperly dished, meaning that the rim is not centered on the hub. There's an easy way to confirm. Flip the wheel over right to left and remount it. If it now rubs on the other side of the frame, than the wheel is the problem.

Lastly is the possibility that all is right, except the frame itself is bent to the side.