Suppose that there is an alien spacecraft travelling towards the Sun. This spacecraft is similar in design, size and power output to Voyager 1 and Voyager 2 as they were immediately after launch from Earth, and is coasting in its orbit (no powered maneuvers taking place).
Also suppose that a budding scientist on present day Earth just so happens to point their instruments (optical telescope, radio telescope, or something else; ground-based or space-based) in exactly the right direction at exactly the right time.
If the spacecraft is communicating at all at this point, it seems unlikely to be transmitting in the direction of Earth.
How far from the Sun (or Earth) could the spacecraft be where we'd still have a chance of detecting it, assuming for a moment that all events line up perfectly for detection? Would we be able to determine that it is likely an extraterrestrial spacecraft, as opposed to some natural interstellar object?