Putt to the fringe of the green from a variety of distances, getting the ball to rest gently against the cut of the fringe. Stand in the middle of the putting surface or putt from side to side. Instant feedback, it is either too hard or too soft. We are not concerned about holing the putt just more concerned about training good speed.
It is vital to hit the ball on the same spot on the putter every time so you know how the ball will react of the putter. A good way to test your contact is to place blue tac on the putter face so the ball can fit between the tac or wrap 2 rubber bands around the putter head either side of the sweet spot. Hit the middle of the putter face and you will have no troubles if you hit outside of the sweet spot the ball will go sideways.