a fair chunk of World Challenge tech work gets done in impound, which can be post-qualifying or post race (generally it's both.) various things get checked at this time. i've already mentioned seals, but actually the list is pretty extensive...
in general the various cars get some sort of stall test. in the stall test, a rubber seal connected to a vacuum guage is used to cover the intake tract of a running engine, which should stall. any odd behaviour on the stall test is reason to look for intake issues.
the cars need to be weighed dry, so each car is observed by a tech inspector while the crew pumps out the fuel tank.
on the scales (which have been leveled using a special platform), the front splitter under the bumper is checked for conformance (there are ground clearance rules and other measurements that control the shape.)
other things can get looked at; for example, at lime rock a couple of cars were tabbed to pull gears out of their transmissions so that ratios could be checked.