1. Check Kayak and Travelocity to get your base price
2. Cross-reference with Skyscanner, Momondo or Mobissimo
3. After finding the lowest price, see if the airline’s official website beats it
4. Check the airline’s foreign website to see if the fare is cheaper in another currency
5. Compare results against alternate routes
6. After getting your lowest fares, play around with your travel dates to see if you can get it any lower
Know that Charter Airlines are not listed on any site.