Not sure if this willl help but...
Haven't tried it myself but I hear that giving each one a wack with a hammer and listening to the noise
it makes can help. I understand that the good ones will make a healthy "ping" when struck.
You can pay a yard to pull one or two to get a sense of the general condition of the bolts and while hauled you can inspect for any opened seam or stress cracking along the keel
Finally I understand it's possible to have the keel x-rayed though as you might imagine this can only give an idea of condition above the actual keel-weight.
I looked at all these options with my previous boat
where the keel bolt heads were buried under glass at the bottom of the integral water
that was to deep to reach into and even start to clear away some glass and too small to climb into. Sold
before I got to that to-do on the list.
Good luck with that and keep us posted!