How would you know if the timing is wrong on a vw golf?

How would you know if the timing is wrong on a vw golf?
basically what happens id you engine will run like crap, stumble and or sputter, while idle, an/or will stumble on acceleration. I have this problem on a 1998 2.0L GOLF, to check if your timing belt is loose, remove the air intake from the filter to the throttle body and remove the timing belt cover held on by two metal clips at/near the top on the sides, grasp the timing belt between the camshaft and crankshaft and twist the belt with your fingers, it should twist 90 degrees or quarter turn and no more.

ATTENTION!: if your timing belt tensioner roller has a damper(looks like it's connected to a piston with a braket), to not attempt to tighten the belt yourself, as the bracket connecting the piston and roller, is what holds the belts tension, not the roller mounting stud/nut, I currently do not know how to tighten the belt, (which the problem I'm having now, after having to replace the mounting stud, as the bolt had been stripped),

However if there is only one bolt, and no piston/bracket assembly simply loosen the mounting bolt, and using a pair of bent needle nose pliers inserted into the 2 holes in the the roller twist until the belt is properly tensioned and tighten the mounting nut to 33ft-lbs

THESE INSTRUCTIONS ARE FOR A 1998 VW GOLF 2.0L 4CYL ONLY, AND ARE FROM MY HAYNES MANUAL.