How hard does it pull? Does it actually "jerk" the steering wheel, where you have to consciously pull it back straight, or is it more of a steady drift or oa drift that gets progressively worse? If you feel it in the steering wheel, I'd have the front brakes looked at. If it's a gentle drift, maybe rears, but I've also seen tires cause this, may not hurt to try rotating the front side to side to see if the pull changes direction.