just use a tape measure on the lip of the wheel, extending it out, and look downwards from the top of the fender. being millimeters are very small increments, you can usually play around with about 5mm and still be the same look. it's honestly somewhat of a trial and error process.

i know some numbers offhand if you know the widths of the wheels, but they are for a stock bodied S14. if its just a usual +20mm front fenders and +30 rear fenders (or +50 maybe), you just use the stock body calculations and add the widebody to it...and you can usually go a little more depending on what kind of flush fit you want. if you're looking to tuck tire and partial rim of the wheel in the rear, you can get away with lower offsets, but it will require more camber and alignment issues.