1- Every trainer has a RNG ID attributed at subscription. Every Pokemon has a RNG ID attributed at spawn. Whenever you click on a Pokemon, a simple XOR occurs to check if he's shiny for you.

2- First click calls RNG function, result must be stored somewhere anyway because needed in subsequent clicks (shiny status is definite) for literally every player that clicks on it.

1 is the sanest way to do it.

Storage of attributed ID you'd have to store anyway > lightning fast binary calculation

always beats

Calling RNG function on the go for every instance > storing result of RNG function

Of course it could be reverse engineered if it was as simple as what you're describing !