Jesus said the 'all has been accomplished' part is what had to be fulfilled between when he was speaking and the end of heaven and earth in order for even so much as a jot or a tittle to be dropped from the law. That's how we know the 'all has been accomplished' part has been done. Because much more than just a jot and a tittle has passed from the law (animal sacrifice for sin, for example).
The Book of Matthew chapter 5:17
Fulfillment of the Law and Prophets
17 “Do not think that I have come to abolish the law or the prophets. I have not come to abolish these things but to fulfill them.[q] 18 I[r] tell you the truth,[s] until heaven and earth pass away not the smallest letter or stroke of a letter[t] will pass from the law until everything takes place. 19 So anyone who breaks one of the least of these commands and teaches others[u] to do so will be called least in the kingdom of heaven, but whoever obeys them and teaches others to do so will be called great in the kingdom of heaven. 20 For I tell you, unless your righteousness goes beyond that of the experts in the law[v] and the Pharisees,[w] you will never enter the kingdom of heaven!