Trim level has a big effect on a used car's fair asking price, since it determines the features, materials, and sometimes even the engine, drivetrain, or towing capacity the car came with.
Two cars that look identical can have meaningfully different value if they're different trims, so buyers shouldn't rely solely on a seller's listing or the car's visible features. The trim level also matters for ordering the correct replacement parts, getting an accurate insurance quote, or pricing a car to sell.
The original window sticker (“Monroney label”) is your first place to start as Federal law requires this to be displayed on every new car before sale, and it lists the make, model, trim, standard equipment, factory options/packages, original price, and VIN. If you still have it, checking that the VIN on the sticker matches the VIN on the car confirms it's original to that vehicle and gives you a fully authoritative record of the factory trim and equipment.
If you don't have the sticker, you can use the VIN: The 17-character VIN (found on the dashboard through the windshield, or on a sticker inside the driver's door jamb) encodes trim-related information and can be run through a VIN decoder to recover the original trim and factory equipment, even decades after the window sticker itself is long gone.
Thanks for reading everybody!
-Paul