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"and this is where it gets tricky. There are 4 batts in the aft starboard basement. They are on a slideout tray so easy to access, but nothing is labeled."
Bill, you could pull the negative (ground wire) off one set of batteries and see if the house electrical system is dead. If so, you know you've removed the ground from the house batteries, etc.
"Logically, one would think that you could tell which batt was a deep cyle and which batt is a starting batt by definition."
House batteries are often 6 volt - not always, but if you see 6 volt batteries, they're likely not starting batteries.
" For example, I have 3 starting lead acid batts and one deep cycle AGM. The 3 starting are Carquest 31S30 Commercial and Farm batts. The AGM Deep Cycle is a Carquest NG31 Platinum battery, which is an AGM deepcycle."
Google 'connecting different type batteries in series or parallel.' You'll find a ton of reasons why this is a bad idea.
"when I connected the charger/engine starter to 2 of the 31S30 batts, the generator almost started"
On mine, the genset starter is connected to the house batteries.
Hope all the above helps a little, Bill. Good luck!!