Typically what you do is work off collision detection events. So if you have a physics collider on the character, sphere/box/capsule - and you get a hit detected, you can check what kind of thing hit the character to know, did I hit the ground.
Additionally - you can do things like in Tick, once per frame send out a short trace/raycast in the direction the player is falling to see if you hit ground. If you do, then you can feed that information into the animation to know if you should be begin playing the ‘prepare to land’ animation.
EDIT:
There’s no ‘wait until true node’ - generally you solve this by having a state enum or boolean that you switch on in tick, one of the states could be ‘falling’. Alternatively, you do something and stop. Then you wait for further input (collision event) and then invoke your own custom event after doing some logic to determine if you need to finish some earlier work you began.
That’s pretty much how the Delay node works internally when compiled, it really is just stopping at the delay node, and sending an event to to our timer callback system. Once the time has been reached it calls an event. That event just so happens to be a function that was generated wrapping all the logic that comes after the Delay node.
Cheers,
Nick