I believe that we've spewed enough radiation that no visual distinction would necessary for detection, at least for the next several tens of thousands of years.
It's true that atmospheric nuclear testing has produced trace elements that were basically never on Earth before 1945, like Strontium-90 [https://en.wikipedia.org/wiki/Strontium-90]. Strontium-90 has a half life of about 29 years, so it will be detectable for a few thousand years or so. Some others last longer.