well waht you could do is guestimate with the tire on..
i would measure with the wheel off the car (tire still on the wheel). TSW doesnt offer every possible offset, so you'll be able to measure this, and then compare your results with the offsets TSW offers for your size wheels.
basically if you get a +21 offset, and you see taht TSW offers +12, +19, +28, +35
you can assume you have the +19 offset. this will give you an idea without having to have the tires removed.
I dont know TSW's offset offering however if they only offer a couple diff ones, this shouldnt be difficult to figure out
