To inspect the centerboard cable, I believe the boat needs to be hauled and blocked, the centerboard lowered and the cable disconnected and removed for inspection. If the boat still has the original cable, I would simply replace it at this time as a preventative measure.
Our cost was around $1700. This was to haul and block the boat, remove the damaged section of the centerboard tube, install a new section of tube (see the above pictures), replace the cable and relaunch. I would expect just inspecting the cable and/or replacing it would be significantly less.
The centerboard tube on my boat is on the port side in the engine compartment. You can see the engine fly wheel cover on the left of the photo and the cabin sole at the bottom. It's pretty easy to inspect.
When we bought the boat, there was a huge glob of epoxy putty on the tube. There was also a wet area at the base of the tube. We didn't think much of it at the time. About a year later I noticed a dripping sound in the bilge and examined the glob on the tube more closely. I found where the water was seeping through the old repair. I knew the seeping would get worse so I started posting and that's when I started this thread.
Take a look at captlar's reply in this thread. That was a huge help for me to understand what was going on.
1972 Bristol 32, #64
"Our doubts are traitors,
And make us lose the good we oft might win
By fearing to attempt."