FHIR api for real time eligibility

I was looking into the available FHIR api. I was looking at the swagger and noticed that appointments has no parameters. I assume that means it returns all appointments ever created? Is that correct? I tried to see, but the swagger demo through a 500 error.

What I want to do, is pull upcoming appointments then get the patients and their coverage connected to that appointment. Then in my system run eligibility checks and push back the correct coverage if I find it.

Is that possible in the current FHIR implementation?