I googled it, but all the results were for omega ruby. I wanna know how to check IVs in Pokemon ruby version (generation 3)
You will have to go to the Legendary Top Pokémon Breeder in the building right behind the Pokémon Center at the Battle Frontier.
His first reading will be of the Pokemon's overall IV total and his second reading will be of the highest individual stat.
First Reading (Sum of all individual values)
Second Reading (Highest individual value)
There's no in-game IV checker in Ruby.
However, you can can check them using an online IV calculator. It's more accurate for Pokemon that are high-leveled and have no EVs.
link to IV calculator