You will need to pull the motor mounts, bell housing and output flange from your current motor for the "new" b20. I am always suspect of PO statements unless receipts are produced (and even then...).
When you part the tranny from the motor you will be able to see the clutch, flywheel,etc. and they will give a pretty good indicator of the quality of the rebuild and the demads that have been placed on the engine since.

When the manifolds come off (assuming they are on it) pull the spark plugs. have an assistant turn the engine over (by hand) while you shine a flashlight into the ports. You should be able to see the piston top pretty well and (if lucky) some of the cylinder wall and definitely the valve head.

Looking at the rocker train will tell a lot too. A good motor will look very clean and you should be able to easily slide a rocker along the shaft (grab two rockers that are not opening valves and squeeze the spring between them). Varnish, Caca or excessive resistance to motion are signs that you should be taking the lump apart.

Pull the sump and give the bottom end a good look and check for nasties in the sump, esp. in the groove around the welded in washer for the plug.

Personally, no matter what I've been told, I count on taking the thing apart for new bearings, seals and gaskets every time (unless I get a good warranty!!!).

Mike!