How to verify check engine light is lit because of fuel injector leak

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Check engine light came on when we pumped poor quality fuel (we think) and subsequently, a rattling sound could be heard when starting the car as it were from the exhaust or thereabouts. Noise has been getting loud, but not too loud. Dealer initially, suggested changing the wastegate actuators.Then it was determined that the wastegate actuators could not be located and the turbo needed to be changed. The rear brake pads also needed to be changed, all in about a 9K repair which we declined. Then the dealer indicated that the wastegate actuators were not the problem and that by cleaning out the wastegates (which they couldn't originally find) they could make the noise go away but to fix the check engine light, they have to change the fuel injector and spark plugs. How do I know what is the right assessment of the issue? Sounds like they are trying to fleece us.