The biggest red flag for a potential buyer would be foundation issues since this is probably the most costly to repair. Things to look for that might indicate foundation issues are:
- Cracks in exterior siding (brick, stucco etc.).
- Cracks in the foundation other than on corners. Corner cracks are common and usually do not indicate foundation issues.
- Diagonal cracks above windows and windows that will not open.
- Doors that are out of jamb and will not open and close properly.
Doors out of jamb 
Interior crack and door out of jamb
- Buyers usually do not go in the attic, but if so… Rafter separation (rafter spread) can indicate foundational movement also.
A lot of homes have minor settlement and lumber shrinkage cracks on interior sheetrock. This is common and does not necessarily mean foundation issue. One door that does not open properly might be caused by a faulty hinge and does not necessarily mean foundation issues.
Inspectors can look at all of the clues in and around the home before rendering an opinion on the foundation. As a buyer you should always have your home inspected by licensed inspector.
