I have no idea how they can tell, but I swear that fish only take flies that the caster has confidence in. Perhaps they see the look on our faces and decide if we're committed to the fly, they'll be committed!
My guess is that presentation and size are the most important characteristics to catching trout in the park. They have to be so opportunistic in their feeding, due to the lack of sustaining foods that if it looks like something they can eat and acts like something that might be alive they'll take it. I know I might be able to catch more fish at certain times, but by sticking with a PA (in yellow for the summer and grey all other times), Yellow Stimi and EHC for dries and a Tellico or Pheasant Tail for nymphs I should catch fish.
Worry more about presentation and have confidence in your fly...the fish can sense it!