Remove the supercharger belt and rotate the pulley back and forth quickly using your hand. Listen for clicking and feel for looseness when you change the rotation direction. It'* probably better to try this when the supercharger is cold, because my experience has been that even with a new coupler, there was a bit of looseness and clicking when I performed the test while the supercharger was warm, but that wasn't the case when the supercharger was cold.
Before replacing anything, it'd also be a good idea to look to see if the snout oil seal is leaking (look on the underside of the snout), and check the supercharger'* oil level and condition.
Note that if your supercharger has 126k miles on it, and it turns out that it needs a new coupler, it might be a good idea to replace the oil seal and bearings at the same time as the coupler, especially if the oil seal is leaking.
'99 Bonneville SLE, white, 112,000 miles
'02 Park Avenue Ultra, white diamond pearl, 88,000 miles