How would a person go about measureing the offset on a set of wheels?

I got TSW blades, and I got them for free so I didnt really care what the offset was, and I noticed they stuck out a little more than my stock wheels, and now that I am going to lower my car, it does not look like they will tuck under the fenders. I dont know much about lowering my car ,(never messed with suspension before). So could the offset on my wheels I curently have be too much to lower my car?