Ice dams form on cold days when heat leaking from inside the home warms the attic enough to melt the snow on the roof.
Damaging leaks may show up in subtle ways, like this line of ice emanating from behind the siding.
Not-so-subtle evidence: icicles growing out of a soffit.
Tannin stains on the siding of this home are unmistakable evidence that lumber in the structure is being drenched with snowmelt.