2002 Cadillac Escalade how do I know if I need service

Every time I take it into the dealer for a simple oil change I'm told there's a list of issues with the car. Last time they reccomended coolant service, transmission service, power steering fluid flush, fuel injector cleaning and throttle body. For over $500. This time the "recommendations" were much more expensive, and they didn't even mention the previous ones. Water pump, rear brakes, belts, tires, alignment air and fuel filters, 4wd service and injector service totaling $2670. This leads me to believe it's suspect. How do I know if I really need this stuff. If I am out of warranty, can't I just go to get an oil change anywhere and wait for problems to arise? Help.