How To Find Out Offset And Width?

Got a pair of wheels however they won't be going on until I can get another suitable 2 that will go well with them. Problem I have is I don't know the offset or width. I tend to buy things first and deal with them later as opposed to doing the sensible thing and getting info first. Anyway on the back there is only markings for the pcd and speedstar (woohoo!) logo. Is there any way for me to measure offset?