I used an angle gauge. I have electronic version from Harbor Freight.
Put it on front driveshaft and measure the angle, then put it on rear driveshaft and take measurement. Now you can calculate the angle between the two.
Then I took the driveshaft off and put the angle gauge on the transmission flange, take the measurement and calculate the angle between transmission and front driveshaft, then same for rear differential flange and rear driveshaft. I the end I found the 3 angles in the drive-train. Based on feedback from SLK owners, the issue is usually in the rear angle. There are two spacer- washers under differential mount, by removing one or two you can adjust that angle.