of moisture in the soil.
If you dig a post hole and get standing water in the hole if you leave the hole unfilled for a day, you have enough water in the soil to just throw the mix in dry. Even half an inch of standing water in the bottom of the hole is enough moisture content to set your concrete.
If you live in a place where you can dig a hole and have it bone dry after a day, then you probably should add water to the hole with the mix.
Err on the side of more water than less. It's nearly impossible to ruin concrete by adding too much water. If you go the other way and add too little water, there's a much higher risk of it not setting at all.