This is really quite simple. Crawl around in there and see how it looks!
Pull up the carpets, or floor mats in the case of early cars, and have a look. You were probably down here already to look at the floor panels, but now look around a bit more. Are there wear holes from the driver’s heels? Are there generic parts-store floor mats to cover up damage?
Move onto the tunnel, rear bulkhead and battery box cover. They should fit reasonably well, with no loose corners, pulled snaps, or frayed edges. They should not be made of wide-loop pile carpet. This is evidence of a home made or budget job.
The door, kick and rear corner panels should be mounted with Pozidrive screws with little counter-sunk flush washers, not those big-flanged ones from the hardware store. They should lay flat against the body, with no evidence of swelling, bowing, or other water damage. If the door panels are wavy, it means that the plastic vapor barrier behind the door was removed by a previous owner and water has gotten into the panel.
If you are looking at a roadster, the long curved trim piece that goes around the rear of the cockpit should be covered with matching vinyl, with a small bead of piping between it and the body. These often come unglued at the corners and try to pull away. This looks simple, but is one of the more annoying things to fix on a B. The trim is held on by little captive T-bolts, with nuts behind the body panels, and several of the nuts are absolute bears to get at…
Look at the dash. The early metal-dashed cars were wrinkle-painted black and reasonably durable. If you have extra holes or enlarged cutouts for non-standard gauges or whatnot, you are looking at having them welded up and getting the dash re-sprayed in wrinkle. Unless you like the holes. I suppose you can always add a clock...
The '68-on padded dashes tend to crack with age, usually at the top, or near the lower corners of the glove-box. There is nothing to be done for this. (Cracks on top can be covered with inexpensive dash-toppers, which can extend the usable life of a padded dash for years, at the cost of having a dash-topper up there.) The glovebox door, if fitted, should stay closed by its own latch and not flop open. There might even be a key for latch, but they are often lost.
Major cracking, tears or cuts in the vinyl dash itself can only be cured by completely replacing the padded dash cover. This involves removing the dash, gauges, center console, switches, etc. The offending dash pad is carved off. The under-dash sheet-metal and impact foam is cleaned up, repaired as necessary and painted for protection. Then the new dash pad is installed, with contact cement, a heat gun, and pop-rivets. Once it is all pretty again, the cutouts for the switches and gauges are cut out and all the bit are reinstalled.
The door caprails should be vinyl covered to match the interior scheme. There should be a “fuzzy” seal or wiper on the back edge, to protect the drop glass, as roll-down windows are called. The door seals should be soft and puffy, with no tears or loose ends. If they are smashed flat from years of use, they are easy to replace. The door top, outside the window, should have a soft rubber wiper to help keep water out of the door. If these are torn, you can get a lot of weather inside the doors, which can lead to rot and warped door panels. If they are dried out and crusty, they can trap dirt, which will cause horrible vertical scratches on the drop glass.
Window seals should be soft and pliable. Dry and cracked ones let weather in and make wing windows rattle. The drop glasses should roll up and down evenly and with only a little effort. If they are hard to move, it could be a lack of lubrication in the regulator mechanism, torn or folded fuzzy channels, i or simple mis-adjustment of the entire mechanism. If the plastic window winders are broken, this is fairly normal. They are ridiculously flimsy and there are chrome ones available to stop it from happening again.