They can't, but they can use common sense. Refs need to yell quickly and clearly if there is any doubt.
Been seeing this a lot too, saw the Beast get correctly pinged for it either this weekend or last weekend - everything is running together for me today - but a lot of players are trying to keep on going illegally.
My understanding of the rule is that if a member of the opposing team touches the ball carrier on the ground at any time, then the ball is tackled and must release. The ball carrier is on the ground when his knees are touching the ground. Someone
My gripe is that (quite often, I think) there is a situation where the ball carrier is tackled around the ankles, and the tackler releases (sometimes becauses the ball carrier rolls on the ground) and the ball carrier illegally goes again, and normally gets away with it. Other times the tackler falls off before the ball carrier hits the ground and the ball carrier continues, legally this time. I'd like to see it reffed a bit more.
I guess the moral of the story is to hold on to the feet a bit longer if your are the tackler.
That, and rolling after being tackled should be controlled more by the ref. One or two penalties in the game and it can stop, rather quickly.