There is no 100% way to get a Pokemon with its hidden abilities, but there is a key way to get it.
In ORAS, at least try to register a tentacool/tentacruel in your pokedex - catching it will help.
Now, it's registered in your pokedex. Now, using dexnav, you can track tentacool around its area and wait until you find one with it's hidden ability. Or, if you have a Pokemon with the "trace" ability, it will tell you if your tentacool has a hidden ability or not.
This will take some time, but it will help you.
Hope I helped! :)