The game triggers a bunch of Messages (events) you can subscribe to in your code in order to react to those messages.
For instance VesselDeltaVCalculationMessage for whenever the game is doing updates to any vessel deltav, or GameStateChangedMessage for when the player enters a different screen (e.g. Flight state -> KSC state).
You could always do any check inside an Update() method of your plugin class, but using Messages is more efficient when you need to do checks on events that are not triggered very often.
To see what Messages are being triggered on certain events you an use the ShowKSP2Events mod:
https://spacedock.info/mod/3375/ShowKSP2Events, or download it with CKAN
This mod displays Messages as they are being triggered, colors them to make it easier to see when the event was triggered and it keeps a count of how many times an event has been triggered so far.
