See if you can find a freshly rebuilt (or even new) turbo to wiggle, then it becomes really obvious when you wiggle a worn out one.
No wiggle at all means a seized one.
The usual signs when looking for any component, oil leaks mean seals are gone, caked and burnt oil means the turbo and/or engine had serious issues.
Eroded compressor wheel means the air cleaner didn't do it's job.