This may be a bit more…
This may be a bit more complex:
"You don't know how to recognize Bip300 blocks, until after there is a Bip300-enabled software release of Bitcoin
So in practice you would first wait for that release, see that is uses OP NOP 5 (or whatever), and then you would (I assume) run a node where if OP NOP 5 is ever used, then the block is invalid"