You can determine a Pokemon's Hidden Power type by using the "Check Hidden Power" option at the Battle Resort.
Here's how to find it:
1. Reach the Battle Resort: This is located in the north-east of the region, accessible from the route connecting Undella Bay and Driftveil City.
2. Enter the "Hidden Power" area: Inside the Resort, there's a specific area with a sign that says "Check Hidden Power".
3. Use the "Check Hidden Power" option: Speak to the person in that area, and you will be able to select the Pokemon whose Hidden Power you wish to know.
This will reveal the type of Hidden Power your Pokemon possesses.