Take it to a dealer for sure. As far as visual inspection goes, check axle seals, look for leaking struts, and any damage or repairs to the front and rear frame rails. Also check the.rear axle for fluid buildup which indicates axle seals are kaput, its a cheap fix but its nice to use as a bargaining chip, mach 1s go through the axle seals long before the fluid if you get shifting them like a pro haha. And check all interior trim for fit, ie. Make sure none are too loose. Mechanicly, losten for squeals or rattles while the engine is running. On the test drive listen for whining or buzzing that changes from revs that could indicate a damaged rear end or trans. Check to make sure the shaker is bolted down properly and isn't moving too much. The parts to fix it are stupid expensive. Make sure any mods are well installed and maintained. Good luck.
2003 Mach 1