try and find out the manufacturer and write down the codes on the shock spring, there maybe a website that will give you the rates. if they only give lbs per inch this can be converted to kg/mm.
you will have to trake the fork caps off and do the same for the fork springs but there is a strong chance the numbers will have been worn off. i have been advised the best method is to remove the springs, measure their length and then take them rates at a suspension shop...this will also tell you if they have sackedd out (no good).
at the end of the day, you can take race and free sag measure ments to show if you need weaker or stronger springs than you currently have.