You could check if the object's speed is zero. Most behaviors have some way to compare speed, alternatively you could save the object's position to some variables every tick and then right above that event check to see if the current position is different than the current. Probably you'll want small changes to count as not moving. Here's an example permitting anything less than 1 pixel as not moving.
global number oldx=0
global number oldy=0
system compare abs(oldx-sprite.x) < 1
system compare abs(oldy-sprite.y) < 1
--- do something since the sprite isn't moving
every tick
--- set oldx to sprite.x
--- set oldy to sprite.y