Still the easiest way to tell where your hotspots are: Watch for melting snow.
Ice damming is especially problematic when expansion occurs, causing shingles, tiles and seams to break. This of course, leads to leaking rooftops as melting ice begins to seep onto the tar paper underlay, and subsequently causes further damage therein. Eventually you can expect the roof to fail altogether.
If addressed immediately and correctly this can be a minimalized issue that doesn't have to cost you an arm and a leg. As any issue, if ignored, it will eventually grow into an expensive repair bill.
The left image depicts a tile roof that unfortunately experienced ice damming in a small amount. This damming was not addressed immediately and before long it was causing tiles to break. The hole that was present, was filled with a porous spray foam insulation, which unfortunately does not resist water.
As a result, the problem persisted, and became worse.