When you look at the camera, take some photos. The important areas are the front, showing lens labels and any buttons or levers, and the left side, where the focus knob is located. Also look at the wind side for wear on the outer edge of the raised circle- a good indicator of usage. Ah, just use your phone and photograph all sides of the camera.
Camera serial number is on the nameplate at top (most times, if not, look below the lower lens)- write this down or make certain it is clear in any photo.
Check focusing in and out- should feel smooth without wobbling. Wind the lever 'forward' clockwise 3/4 turn, then back until it stops. Do this a few times for feel. After winding forward and back, fire the shutter; repeat. Set shutter to 1 second and listen/watch for how long it stays open- sticky shutters are common. Look at the lenses for obvious scratches, etc. Open and close the focus hood on top a few times, feel for binding, etc. Check how the back aligns with the main body, and open/close it a few times. Sit the camera on a flat surface and see if it rocks on its 4 little feet.
After this you can start doing research and get closer to a possible value. Sometimes the camera will need work before it is usable, so this needs to be figured in. But not every Rolleiflex has to be overhauled before use.