Just watch Ken’s feet while you’re executing. Try it with stand forward, or roundhouse. It’s too hard to determine with f+forward in my opinion. If you’re doing it right, Ken’s feet will move very slightly forward before throwing. One way to practice is to set the opponent to crouching all guard, put him in the corner and low short. Normally, for this tick throw you have to walk forward and throw, but if you kara throw with stand forward or RH, there is no need for walking up.
As for the kara DP, you really can’t tell if you did it right unless it connects. Since you are cancelling the first few frames of a normal, the only animation that comes out with a successful kara DP is Ken about to put his hand on the ground. It’s way too fast, maybe if you have slow-mo, and don’t blink while you’re training. Or maybe if you’ve had 100 coffees.
The motion I use is forward, down, down-forward RH/jab(:dp: :hk:/:lp:) The motion you are using is no good because you are pressing d+MK, then df+LP, which is too slow for a kara cancel. To practice, just pick a shoto, and low forward xx jab dp, kara dp mid-screen(this is an important combo for Ken) until they get close to the corner, cross them up, and repeat. Good luck, don’t give up!
Also… try searching the forums for “kara throw”, “kara srk”. There is some good information that has helped a lot of people learn.