How do you tell if the belt tensioner assembly is bad?

How do you tell if the belt tensioner assembly is bad?
How do you replace it? I am still having some clattering from the tensioner pully after changing the harmonic balancer.
I am stumped again, I installed the dampner, pushed it on good and tightened the center bolt. The pulley appeared to be inline with the others, so reinstalled the belt. Started the engine and ran for 10 min. idling, then went down the road for a mile or two and back. After I got back home, I heard clanking, the belt tensioner pulley was jumping some? I took the pulley off, the bearing seemed pretty good, just a little rough, was able to inject grease into the bearings. I finally replaced the pulley with a new one, then the problem is still there???????
The tensioner pulley jumps erratically with the clanking. I don't know if it is the tensioner assembly or some other pulley grapping the belt at times?
So far the belt is not being damaged yet. The belt tensioner assembly has good tension and strong, with no slack/slop in it but it is jumping on/off as the belt is rotating.
Do you think maybe the pulleys are still out of alignment some kind of way? Is it possible that the dampner could push in too far or does the shaft key stop it properly?
Please send me some help, I don't what to replace a $125 belt tensioner assembly and the problem continues.

RESPONSE:

With the serpentine belt off, the tensioner pulley arm has zero sideways movement when pulling on it. It also has a substantial tension and recoil with the belt off. I just notice that the tensioner groans when raising it and on releasing it when installing the belt, it goes to a point and stops, then you can push it downward by hand maybe another 1/16" at times.
I am eyeing the pulley alignment and had someone with mechanical experience involving pulley drives on heavy equipment, take a look,listen. He thought everything looked normal and was rotating well now, after cycling the a/c compressor clutch off/on, trying to get some tensioner pulley chatter.
So I don't know if I can have full faith in it right now. I will put some miles on the car tomorrow around town and see if it reappears and eats another belt.
I cleaned all the drive pulleys to make sure there wasn't some of the old torn up belt debris/crud in the grooves, either making it prone to jump or grab while rotating, (a long shot effort) ???
After running for 10-15 mins. warming up, so far all is ok, I revved the engine to about 2000 rpm, up/down and no noise at all, no squealing or chattering.