Apparently, with sufficiently powerful optics and a bit of planning, the answer is “yes”. Astronomers are now unofficially competing to see who can spot it the farthest away. 😉 Admittedly, they’re really seeing the Falcon upper stage more than anything else (it’s big and covered in reflective white paint after all), and currently the record stands at 2.5 million kilometers:
This was taken yesterday with a 0.8 meter reflecting telescope at Celado Astronomical Observatory in Italy, and the bragging rights go to Riccardo Furgoni and Giancarlo Favero. The apparent magnitude of Starman was about 19.3, so there is definitely equipment that could still spot it. And that actually makes this a very useful thing to be doing — tracking the Falcon upper stage and the Tesla Roadster is fantastic practice for tracking potentially hazardous asteroids. It is, after all, on exactly the sort of Earth-crossing orbit we ought to be worrying about.