You cannot repair dampers of this type. To ascertain their true condition, you need to take them off. You could try a bounce test. You need a couple of friends to help you get the front end (one side at a time), to move up and down, to the greatest extent you can. Another way is to drive and brake hard and then immediately drive off again and brake hard again. Either method will give you some idea, but neither will be as effective as getting them of the car. If in getting them off you allow them to be either horizontal or worse, you will need to bleed the gas from the cylinder. To do this mount them upright in a vice and push the damper top all the way down. Allow it to return to the top and then do it another 5 times.
Roger