What are the first signs of turbo failure in a turbocharger?

As a keen observer of automotive performance and maintenance, I'm curious to know: What are the initial indicators that suggest a turbocharger is beginning to fail? Are there any audible changes in the engine's sound, such as an unusual whistle or rattle? Do drivers typically notice a decline in power output or acceleration, especially under heavy loads? And how about exhaust smoke - is it a common occurrence in turbo failures, and if so, what color might it be? Any answers would help me in early detection and prevention of potential turbocharger issues.