If you're using the physics behavior there is an expression for mass and X an y velocity which you can combine to get the speed with distance(0,0, vx, vy).
If using another behavior then most have an expression for velocity or speed as well. Or if you'd like to calculate the speed with events you can if you keep track of the old position of the object.
Set text to distance(Sprite.x,Sprite.y,oldx,oldy)/dt
Set oldx to Sprite.x
Set oldy to Sprite.y
Outside of the physics or chipmunk behavior's an object has no mass so there's nothing to calculate.