How do I know if I need a tune up?

How much should a diaganois cost on my vehiche.I have 90,000 miles on the vehicle and no major repair.I change oil ,air filter and fuel filters on regular basis and use premium shell fuel.

If the Check Engine light is on diagnoses should begin there. If the light is not on the diagnoses would be more general. In either case, I would suspect 1 hour of shop time should give a good idea of what is going on.