The best way to find out is to place a straight edge on the face of the wheel/tire. Take a mm scale or rule and measure the gap from the inside of the straight edge to the inner fender. The mm gap will determine the size spacer you'll need. Remember to compensate for camber if it applies to your car. Take the measurement where the wheel is closest to the fender.
H&R's hubcentric spacers are top notch, and range from 3mm up to 65mm
To add, a standard scale or measure will probably work, you'll just have to convert it to mm. *25.4mm = 1 inch
12 Black GT500 SVT PP l VMP TVS l JLT123 l O/R X l 28x11.50Rx15 M/T Street R's l BMR Suspension l Recaros
03 Sonic Blue 3.8 / 17 Magnetic Fusion Sport 2.7 EB