I’m a bit late to the party but in addition to what has already been suggested, another way to tell is that if you hover over a property in the output, you will see a tooltip with its input form, e.g. EntityProperty["Airport", "Cities"].

Looking into this a bit more, I am now both intrigued and confused.

Building on Rohit's advice, I tried using //CanonicalName on some other *Data objects such as WordData. To my surprise Mathematica returns

CanonicalName: AmericanSpelling is not an entity

... So, there is clearly something different about AirportData and WordData. It seems as though AirportData is a proxy object for Airport, but I am not sure how I would know that, and what that implies. I am curious to understand.

Trying to retrieve all of the properties also generates an unhelpful message, but at least it does return the values for all of the properties.

AirportData["KATL", #] & /@ (AirportData["Properties"] // CanonicalName)

EntityClassList::noent: Hartsfield-Jackson Atlanta International Airport is not an entity type.

Hi Andrew,

AirportData["Properties"] // CanonicalName

(*
{"AlternateNames", "Cities", "Coordinates", "Countries", "Elevation",
"EntityClasses", "FAACode", "IATACode", "ICAOCode", "Latitude",
"LongestRunwayLength", "LongestRunwayWidth", "Longitude", "Name",
"NumberRunways", "OfficialName", "Position", "Uses"}
*)