Upvote:2
You can set up 'fare alerts' in popular apps/websites such as Momondo. Skyscanner and Google Flights. These may or may not capture discounts or flash sales. The other alternative is to subscribe to individual airlines newsletters, which may make you aware whenever a discount or flash sale is offered
Upvote:3
You can use Google Flights for this. If you search for a certain route and dates, you can then activate alerting and monitoring for prices on that route (on the result page there is a checkbox called “Track prices”). However I’m not sure if this only appears if you’re logged in with a Google account.