One way is also to check, if the flight has availability in fare bucket U. This can be done e.g. by Expertflyer (paid) or using Amadeus API (free, but requires a bit of programming).

For HEL–SIN route that can be a bit tricky. My information is now a 1 ½ years outdated, but back then it was quite common that even plats didn't go trough the upgrade queue untill T-37.