There's a get object speed action
http://hutonggames.com/playmakerforum/index.php?topic=1182.0but it will probably only be helpful for telling how fast an object is moving overall.
What you could do is put a check via a float compare into your jumping section of your FSM to check the Y value to the previous Y value and if its negative, then start the falling animation.
eg: after pressing jump, you transition to your state that plays your jump animation then to the next state
that has a "get position" action and a float compare
Get Position Action: Store Y value as Yspeed variable
Check Enable: Every Frame
Float Compare Action:
Float 1: Yspeed
Float 2: Yspeed
Less Than: GoToFallingState
So each frame it checks your player's Y position if its less than its previous value and if so, transitions to your falling animation state.