At a home inspection in Stillwater, MN, I found evidence of frost. Finding it is easy during the winter because the frost forms on the nails from the shingles. In the summertime I look for rust on those nails, as well as darkened wood where the moisture has caused damage. Moist air in the attic can cause a few problems; most notibly damaged wood and compromised insulation.
The cure is not too pricey, smewhere around $1,700 – $2,000 for an average house. To read an incredibly smart and funny article about sealing attic bypasses, click: Here