One way is measuring total flights, number of passengers carried per year etc vs number of incidents and accidents of any kind, or passengers lost as alluded to above. Then there's mitigating factors like "airline A" lost those passengers because of a design flaw the manufacturer knew about and didn't correct (EG the UA 747 cargo door over the Pacific).
Airfields flown to are indeed important, look at Ryanair for example, flying over 100 million passengers a year into class g, procedural airports in remote and poorly equipped airfields. Never lost a passenger either.
And you would have to say the big US majors must be doing something right, most movements in the world on often antiquated equipment, and their ATC often aren't using modern equipment either. That's the culture itself amongst employees working right.