Not by calculation, but by observing the depth or level at which moisture in the soil freezes. The term frost line as used in architecture is really a level or depth, that varies with location and it is important that foundations extend to a level well below this, so that frozen water in the soil does not cause the foundation to heave. The term frost line might also be used in weather reports, and would probably mean geographic areas or elevations in mountains, beyond which freezing conditions occur.

If there's a tree nearby, it could be roots growing under the paving and lifting it.

If you live in a region where it gets cold enough in the winter for the soil to freeze, it could be what's called "frost heave".

It is one form of mechanical weathering. Freeze/thaw cycle. Frost heave.

Either by that action of Ice made from compacted snow (Glaciers), by frost heave, or by the action of meltwater.