Some are more adept at reading body language than others, and this skill increases with training. But the beginner learning Delayed Sword will have a very difficult time. Maybe do an experiment and let an attacker come at you with a grab, push to your chest, a right palm heel, or even a punch, all with the right hand and with the same body movement. Try to guess. Maybe you can. Most can't, especially a beginner. When you parry a punch or palm heel into yourself thinking you were pinning a grab, the results will be pretty clear.
So what do we teach a new student with Delayed Sword against a grab -- we are teaching the student to stand there, allow the attacker into the danger zone, yet stand with your feet together, until they have a good hold on you. Logically, it doesn't make sense.
Certainly, someone can grab you when your head is turned, or you aren't paying attention for one reason or another, and so it is important to learn to extricate yourself from a manipulation, but that is a more specific level of training to me, than attempting to intercept an inbound limb regardless of what they intend to do with the hand that is on the end.
Just a different approach of thought.
Derek