An unmodified FireRed or LeafGreen English 1.0 game will say "Game Freak" when it boots up, before the Pokemon battle, and Pidgey's Pokedex entry will say it's the "Tiny Pokemon"

An unmodified FireRed or LeafGreen English 1.1 game will say "Game Freak" and then the word "PRESENTS" will appear below it, and Pidgey's Pokedex entry will say it's the "Tiny Bird Pokemon"

Alternatively, you can open the ROM in HMA and look for this byte at the very beginning.

It will be 01 in 1.1, and 00 in 1.0. You can use this method to check the version of any GBA game