To paraphrase a former Supreme Court justice:

"I can't describe it, but I know it when I see it".

Some factors are: regularly bicycles because they want to, takes into account the current conditions and adjust riding style (road conditions, traffic volume on street or MUP like SART, closed course at a race, weather condtions, time of day/nighttime, etc.). Defiantely not type of bicycle, its composition, or the clothes the rider is wearing. And you have to be able to fix flats on any wheel - practice at home until you get comfrotable with it.