well i would start with what 'wollysheep' said.. tyre pressures.
have you hit a kerb or been down any big pot holes recently ?
then a few things to check..
does one wheel seem further back than the other? .. mabe you could measure with your fingers the distance between the tyre and the car body on both sides.
mabe you could jack the car up and check for worn bottom ball joints or worn bottom arm bushes.
does one wheel seem to tilt out at the top , more than the other side?
does one side suspension seem lower than the other?