The intended way to know is that there is a little icon displaying the tail fin of each operating carrier.
However, Finnair did not make icons for all operating airlines. This means that the only visual cue is flights were no icon is displaying - then you know it is a bad flight.

And of course, you can click on every option in the list to discover flight numbers and operators. But that is hiding vital information IMHO. The Finnair statement is false. The operating carrier is not shown. There is an option for savvy bookers to find it, but is isn't shown.