As some of my flights have been canceled I'd like to find out how the airlines are operating (easyJet, Smartwings, Czech Airlines, ...). As they don't put much information, is there a possibility to find what destinations they're currently serving from an external resource, e.g. https://www.flightradar24.com/ ?
Flightradar can give you good historic information about flight history. From there you can see if the airline has stopped some flights or is running a reduced service.
The most reliable source is, of course, the airline. Most airlines will have a COVID-19 help section:
This will give you an idea of the airlines plans for the future.
As mentioned in the comments:
The airlines are the people to ask, but often they don't know because they're waiting for governments to make announcements. Both easyJet and SmartWings have sections on their web sites dedicated to coronavirus questions. If what you want isn't there, it's probably not available.
A lot of things aren't certain right now but are just guesses.