Proper tension on a bushing like that can be measured-
my Haynes manuals give a spec like 42mm or some such for the distance between the bushing washers when it's all tightened. Do I always measure it? No, but I've got a little experience looking at them. For a person who is new to it, follow the book recommendations.
I've also found that some sway bar bushings are quite hard to compress and require jacking the car one way or the other to get enough length to start the nuts... anyone doing this job will see what I mean. You want to get the sway bar end angle as close to horizontal as possible when installing the new bushing. Sometime that means jacking up the opposite side wheel.
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K

Sometimes cracks can be superficial, and don't necessarily mean the bush is finished. You can do a certain amount of testing using a very big screwdriver, crow-bar, or something like that. Wedge the end in between the components and see if you can detect much movement. Try it both with the weight on the car, and also with the suspension partly dangling with the car raised on its jacking point.