Its very easy. The 0.35 or 0.38 etc means K = 0.35 kg/mm. For every 0.35kg load you put on the spring it will travel 1mm. Take a 5kg waight, put it on top of the spring and see how far it travels in mm. Divide the weight ( 5 ) by the travel in mm and whala, you have the k factor. The more weight you use the more accurate will the result be.
IF you find it difficult to compress the spring ( it jumps sideways ), go to your fishing buddies. Take the scale they weigh their record fish with. Hang the bike spring to the garage roof, hook the scale at the bottom and pull. You can measure the distance travel for whatever kg you pull.
Good luck.