On our boats this can be difficult, there's enough glass wrapped over the stringers that I think the boats would actually work fine if theoretically you could suck the wood out, the remaining glass would suffice. (similar to what Malibu & Mastercraft do) Generally they would be found when replacing a floor that has rotten, on glass floors (1988+) the floors are not likely to need replacing. Other ski boats find the problem because the motor mounts are lag-bolted into the stringers & the bolts are now all stripped & won't hold. All A/S boats use a unique aluminum engine bed which is through-bolted to the stringers with locknuts. I recently cut a large hole ahead of the pylon on my '88 Advance to inspect it & found some rotten bulkheads that were there to block foam during production, I drilled into the stringers to satisfy my curiosity & found solid wood core. ( I filled the test hole with reduced resins) The only way to really tell is by removing any bolt or screw that penetrates the stringer & probe with a sharp ice-pick type tool, if the immediate area where the screw came from is soft you then need to bore a test hole at least 2-3" away & retest. I have a 1" hole saw with the pilot drill set really short to do this. On many of the go-fast boats I work on this is a common procedure when replacing the transom, I will bore every 6" until I find solid core so determine how far I need to replace stringers.
Unlike other brands I have NEVER heard of a catastrophic failure of an American Skier boat do to rotten stringers. The worst I am aware of are (2) different 20+ year old boats having lifting ring "pull-outs" both at the transom end. Hope this helps...RonT
