Musk's Roadster is now Russell's Teapot?
Small objects in space are hard to detect and characterise. Compare the tesla with another small solar system body: 2015TC25, It is currently about 0.3AU (3/10 of the distance from Earth to the sun) and it has a magnitude of about 30. That is perhaps detectable if you know exactly where to look. It would not be "discoverable" at this distance, with current technology. We know about because it made a close pass of Earth in 2015, and it was discovered during that pass. 

The Tesla could be discovered if its orbit takes it close to the Earth. Observations of its spectrum would indicate it is not a regular asteroid, The cherry red paint would mark it as being utterly different from natural asteroids.

How close depends on what technology you have. By comparision with 2015 TC25, it might be observable from 0.3AU, if you have enough time and money to spend.