This is what I did, get a ruler or a caliper and take these measurements
1. Measure the length from the center of the bike to the chainring
2. Take out the BB and measure the spindle length (or see if its stamped on there somewhere), then divide that number by 2
3. Measure the distance from the center of the wheel to the cog
With the first two measurements you can figure out how much offset your crankset puts in your chainline, with the difference known, you can mess figure out what spindle size would give you the same distance found in measurement 3.