If you click on the province, you need to look at morale, it usually rebels anywhere under 30% morale. There will be an icon, a white fist on a red background if I remember correctly, which indicates if there is a chance of rebellion. You can also look at the province details and it will show you the rebellion percentage chance.
To counter this, a province must have a garrison strength of at least 6 strength. You can see what the garrison strength is on any given province by clicking on the details to see. If there are no troops there, it will be 0/6. You can check each units strength by clicking on their details too. So simply put a unit/multiple units totalling at least 6 strength in the province to stop a rebellion.
Rebellions happen at day change
