Luckily for the buyer, tilt-shift lenses are manual-focus (at least, all the ones I've ever seen are), so checking focus is the same as any other old lens. That is, verify smooth focus and smooth aperture throughout both ranges.

Regarding the tilt and shift movements, you're basically just checking for range of motion (the limits of of both motions are usually marked on the lens), and for smoothness of motion. As long as the motions don't feel gritty or sticky, or at the opposite extreme, overly loose or sloppy, and as long as the tension locks keep the movements in place, there's not more to judge on a tilt-shift lens.

The dial knobs for the tilt and shift movements tend to be small diameter, and the gears driven by those knobs even smaller diameter. If the lens is configured or oriented for rise/fall shift movement, expect it to be difficult to rise the lens using the knob alone, and expect the lens to shift back down if you let got of the knob. Personally, I help the rise movement by lifting the lens body with my other hand while rotating the shift dial, and then lock the lens down.

Similarly, depending on the specific tilt-shift lens, something the tilt movement is hard to tilt the lens upwards with the knob alone; in that case, it's fine if you need to give it a little help.

A final check that is easy to forget is to double check the lens rotation movement(s) is (are) smooth, and that you get full 90° range of motion between portrait and landscape orientations, and still have full tilt/swing and shift movements in both orientations. Certain PC lens and body combinations work, but experience interference between the prism overhang (on DSLR bodies) and the fully-shifted lens. Luckily, those combinations are well known, so you can search the internet in advance for interference with your body and chosen lens combination.