State the connection you need
Separate hands-free calling, music playback and phone projection in your checklist. Wired and wireless connections are also different questions. Do not assume that the presence of Bluetooth or a USB socket establishes all of these capabilities.
Bring the right equipment
Bring your phone and an appropriate cable if you intend to use a wired connection. Ask permission before pairing. With the car parked, follow the vehicle's connection instructions and confirm which functions actually appear. Keep the phone secure and avoid entering sensitive information into an unfamiliar interface.
Check a realistic task
Try the task that matters to you, such as selecting an audio track or opening a supported navigation interface. Check the vehicle's audio controls as well as the phone screen. If the connection fails, note the device, cable and message rather than immediately deciding that the car lacks the feature.
Remove your information afterwards
Before leaving, disconnect your device and remove its pairing from the vehicle with the seller's permission. Remove the car from your phone's saved connections if appropriate. Do not leave contacts or accounts in a car you are only viewing. Your notes should say exactly which device and connection method worked, without generalising the result to every phone.