We check fixed sample trips on each route through Google Flights using SerpApi, and save every result with the time it was fetched. A fare counts as a good price when it is at or below the low end of the typical range for that trip, and, once we have 30+ days of our own history, when it is in the cheapest 20% of what we've seen for that route and travel month.
Google Flights includes most airlines on these routes, including Southwest, Alaska/Hawaiian, United, Delta and American. Prices are for 1 adult in economy and can change at any time.