Teak decks, if previous owners didn't over-sand it or use chemical teak cleaners periodically, which does the same thing as sanding--- it removes the uppoer wood cells. are simple to maintain. So even if the seams between the planks is pulling loose from one side or both sides of the seams, or deck plugs over the screws are missing, it's easy to repair. It's time consuming to repair, but the work itself is very simple.
If there are seams pulled away from the groove sides, it's a pretty sure bet that water has been getting down under the planking. Whether it has been getting down into the subdeck core is more difficult to determine. If in walking around on the deck you feel softer spots or more give under your feet, the chances are good that the subdeck core is suffering from rot. The typical deck construction had the teak planks laid over the subdeck, which is generally a fiberglass-plywood-fiberglass sandwich, with the bottom layer of glass much thinner than the upper layer. If you can get to the underside of the deck, a moisture meter will have a lot less material to "shoot" through than if you try it from above.
I don't know if there is a "tap test" that can be done on a deck to reliably indicate problem with the subdeck.* You'd need to talk to a shipwright or surveyor to find that out.
The three photos are from the Grand Banks owners forum. The first shot is a plug that a fellow took out of his deck to install a new fuel fill. While this is a Grand Banks, the deck construction is probably typical of what you'll find on most cruisers with teak decks. How the thicknesses of the fiberglass and plywood core layers of a GB compare to other boats I have no idea.
The other two photos I included for interest. This deck is actually a synthetic product from England called Tek Dek. According to the person who redid the deck on his late '70s GB using this stuff, it's holding up very well, looks great (if you like the look) but is very, very expensive. Almost as expensive as using new teak he said.
