how to measure for wheels?

how is one to measure the back spacing,offset,ect. for a wheel? I'm in the market for some moons,and i am wonding where to measure? Are there tools for this? Thank you

lay the wheel face down. take a straight edge lay across the back of the wheel, you may have to dismount or let the air out of the tire to do this. measure from the straight edge to the mounting surface of the wheel, this is your backspace. i have seen tools for this, and i've seen people make their own tools to do this, i cant say where to buy them though. hope this helps.

thank you Jeremiah, that was easy!