To get a general idea of where your chainline _should_ be, run a piece of string from the cog to the bottom bracket shell. Then measure the distance between the chainring and that piece of string. Next, measure the space between the inside of the crank arm and the bottom bracket cups. If the first measurement is less than the second measurement, you should be able to replace the bottom bracket without having to swap the crank as well.

After you've done your initial measurements, remove the crank arms and the bottom bracket. You can determine whether your BB is French by noting the direction the fixed (drive side) cup is turned to remove. Once the bottom bracket is removed, determine the bottom bracket spindle length. Sometimes the length is stamped on the spindle itself, if not then measure.

Subtract your "ideal" chainline measurement from the spindle length measurement, and that gives you the new spindle length required to correct the chainline.