A turbocharger is a compressor that is mounted on the exhaust side of a vehicle and is intended to dramatically increase the horsepower of the engine. As the engine's rpm increases, the turbocharger forces air into the engine in order to increase the power. This is known as forced induction. If a turbocharged engine is not maintained properly, the turbo may fail, leading to a costly repair bill. There are several symptoms of a bad turbo, though not all of them are caused by a just a bad turbo.
Test drive the vehicle and see how the power delivery is responding. A car that has a bad turbo or turbos will still run, but it will not produce the same amount of power that it does when the turbos are working the way they should. If the car's engine is producing much less power than it did before and accelerating slower, but still running, the turbos may be failing.
Look for a "Check Engine Light" on the dash. In a modern turbocharged car with sophisticated on-board diagnostics, a bad turbo will usually cause a check light to turn on. Keep in mind though that a bad turbo on a turbocharged car is not the only issue that will cause a check light to come on. If there is a warning light, a shop with the proper diagnostic equipment will be able to use a computer to determine the problem.
Find out if the engine is smoking excessively from the tailpipe. A turbo that has bad internals or blown seals will usually leak oil into the exhaust system, which then burns off, which produces smoke from the tailpipes. The smoke will often become worse as the rpm on the engine rises.
Check the boost gauge if your vehicle is equipped with one. A boost gauge is mounted in the interior and reads the amount of boost that the turbo is producing. Not all cars have a boost gauge, but if your vehicle has one and it is not reading very much boost -- which can be anywhere from 9 psi to 14 psi for most normal turbo cars -- when the car is being driven, the turbos are likely going bad.