We went in having researched first (brand literature and reading here), and while our dealer seemed to know all the specs I did, I'm sure he would have sold us even if we were WAY over on our vehicle limits. I bet this is the case anywhere, they are all salesmen first, even if they do know their product.
It's a safe assumption on TTs though that tongue weight will be 10-13% of trailer weight. Ours is in the ballpark of 3000# trailer (loaded) and 300# tongue, plus another 80# or so for our WD hitch, so tongue is about 380# for us, our little Jeep has a carrying capacity of 1050#, so with Myself, Wife, Dog, and tongue, we have only about 50lb to work with in the Jeep, but that's fine for us, any gear we need is in the trailer. We do plan on a better tow vehicle few years down the road, but we are within safe limits and happy with it for now.