The short answer to your question is that it is extremely rare for a boat to sink due to blistering. If you pull and inspect your boat on any reasonable periodic schedule it will be quite plain to the eye if blistering is present. A surveyor can yelp you too if you search out one with a trained eye for that sort of problem. If you are looking at any boat older than 1990 you can almost bet on some blistering. Structural blisters are large and tend to be quite obvious to a trained eye.
Most blistering I have delt with shows as a visible pox of small bumps where the gel coat is popped loose from the underlying matrix. These bumps are typically anywhere from ocasional to very dense and range from BB size to abut tripple that size. West Marine has a West System video that will show you how to handle these. In short, grind them out into a region of good confirmation, dry them extensively, fill with an appropriate epoxy filler and fair flat. The important step will be to remove ALL antifouling paint, dress the surface and apply a sealer such as INTERPROTECT 2000 or other sealer rated against blisters. We were fortunate that the yard where our boat was would permit sand blasting to remove all previous paint. The added effect is that it will expose most surface blisters. You could also get your hands on a device called a peeler if the hull is in bad enough condition to warrent a total replacement of the gel coat.
You must plan in advance what antifouling paint to use since effectiveness of its bond to the sealer may require timing its application to the hardening of the sealer. The Interlux and Pettit gurus turned out to be really helpful on this for us and even consulted one-another to suggest the best choice for us. Go figure!
Some boats may have some really impressive blisters between the laminations of glass. These are not cosmetic and will require some re-building of the damaged area. We found 8 such major delaminations on our bottom job of a 1984 vintage boat that spent 100% of its life in the water in the tropics. (worst case for a boat). These were seperations in a layer about 1/4" to 38" deep and over 6 to 12 inch diameter. Pressure inside was enough to shoot trapped fluid up to 30 feet when lanced with a drill bit. I cut these out with a right angle grinder and faired them out up to 24 inch diameter looking for totally dry matrix and to leave a 15 degree tapered edge. these were then built up using West epoxy and glass. Once faired, treat as above with seal layers.
Really big blisters can be detected with a moisture meter or you can sight the hull in varying light levels and direction every time you are there. If it looks like it's sticking out - it probably is.
One note, the rudder was so bad I have stripped it for a total do-over right down to the stainless skeleton.
If you are dealing with an old boat and you are not confortable with your own abilities to inspect then I strongly suggest a survey. condition will depend greatly on the care the boat had.
OLD: any glass hull made before the causes of blistering were learned and the manufacturers reacted. Better compaction of the matrix, vacuum bagging and better resins imporved the situation. I am not sure of my dates but I think things got better in the late 80's to mid 90's.
The break came when they changed from polyester resin to vinylester resin which is markedly less prone to blistering. Many owners also never sealed the bottom, or maintained the sealer. Sealers have improved too. some manufacturers never got the message and continued to make polyester resin boats.
View my photo log here on the sailnet (Nicholson58) where I have a lot of pictures showing a badly blistered bottom and some of the tools. We scraped off the soft VC tar layer; blasted the underlying friable other paints; ground out all blisters by hand with a carbide grinder, filled using West System; faired the entire hull with a 40-grit flexible wheel, applied 6 coats of Interprotect and finished with 3 coats of VIVID yellow. If I did it again, I would start with a peeler.