FBinNY 

Many assume that a wheel off center in the seatstays means the rear triangle is asymmetrical to the centerline (assuming wheel is dished). That's the most likely cause and needs to be checked by the string method.

However there's another possibility and that's a wheel that is not lying in the plain of the frame, ie not vertical when the bike is. That's more common on new bikes because it only takes a tiny error in the heights of the two dropout slots to produce a visible error - the rim will move by 4-5 times the error in the dropouts.

I detailed a method for checking this on another similar thread using a bubble level, but it takes a sensitive level (they vary). It can also be done on a flat table or floor, but probably needs two people. Tools required include a level and sliding tri-square, or other height gauge.
1- remove the rear tire and mount the wheel with the bike on the floor to ensure it's fully pocketed seated at the top of the dropout. If bike has horizontal dropouts center it in the chainstays, and tighten.

2- lay the bike on the surface with the seat tube resting on 4x4, or two equal height supports so it's parallel to the floor. Confirm with level, of height gauge at each end.

3- support the headtube or rear triangle on shims so the bike is also level in the other axis. Confirm by checking that top tube is horizontal.

4- with the bike horizontal, the rear wheel should be also, use height gauge to measure bottom and top (if in riding position) of the rim which should be equal, or lay the level on a tangent across the rim in what would be vertical if the bike was upright and confirm.

5- optional, remount rear wheel cheating so it centers on the bridge and remeasure to confirm that this solves the skew.

You repair non vertical wheels by carefully filing the tops of dropouts, but go slowly and check often because the rim moves 5 times more than the axle, so the OP's wheel would need less than 1/32" adjustment (if that's the problem).