Ask for specific photos and details
If you have doubts, ask for specific material.
For a guitar: front and back of the headstock, serial number, frets, bridge and any declared damage.
For an amplifier: rear panel, speaker, serial number, rating plate and relevant visible components.
For a synth: powered-on display, keyboard, rear connections and a short video showing a few controls working.
The aim is not to interrogate the seller, but to verify that the photos, description and actual availability of the instrument are consistent.