Create two lists
Keep original equipment and later additions in separate sections of your notes. Include a third section for items whose origin is unclear. This avoids treating an attractive accessory as proof of the vehicle's original configuration.
Ask what the installation involved
For a later addition, ask when it was installed, which components were changed and who performed the work. Request any installation or inspection documents that the seller holds. Do not infer correct installation from a tidy-looking switch or an absence of obvious warning messages.
Check integration, not just appearance
Ask how the addition is operated and whether it affects other functions. For an added screen, for example, check the functions you need rather than judging it only by its size. For safety-related or electrically complex changes, use a suitably qualified workshop to assess installation and compatibility.
Describe uncertainty honestly
If the origin of an item cannot be established, write that down. Avoid calling it factory-fitted simply because it resembles an original part. Before a handover, agree which accessories and any related instructions will remain with the car. Keeping the original specification distinct from later work makes both a buying decision and a future sales description easier to understand.