Where should the soil sit around a newly planted tree?

How much settling around a newly planted tree is normal before it becomes a concern? I’d like to understand how to judge the root flare against the finished soil grade, especially if watering exposes more of the root ball or leaves the trunk sitting lower than it did at planting.

I’m wary of solving a low spot by just adding soil or mulch around the trunk. What signs suggest the root ball has sunk too far or the grade has washed away, and when is it better to have an arborist assess it rather than trying to correct the soil level?