If I understand correctly you would like to know if the speed is decreasing, to play a different animation..
You could give the player sprite a varaiable Lastspeed and every x seconds check if this last speed is higher than current speed
system every x seconds
system compare two values player.lastspeed > player.speed
player animation deceleration is not playing
- player set animation to deceleration
and add an event AFTER this one to set player.lastspeed to player.speed