John Gill speculated:
The report of this was brought him very probably by one of the maids which attended her to the city; for it was hardly to be thought that she should go thither alone.
But this doesn't seem particularly authoritative or to rise much above a WAG. The text itself does concern itself much with this particular detail. Presumably the reason it skips over this is because Yaakov's hearing of the rape was not as surprising as his unusual reaction to it. As the NET Bible notes:
The expected response would be anger or rage; but Jacob remained silent. He appears too indifferent or confused to act decisively. When the leader does not act decisively, the younger zealots will, and often with disastrous results.
We do know that Yaakov's sons heard of the event independently:
And the sons of Jacob came in from the field when they heard it; and the men were grieved, and they were very wroth, because he had wrought a vile deed in Israel in lying with Jacob's daughter; which thing ought not to be done.—Genesis 34:7 (JPS)
It's not entirely clear, by the way, who "the men" refer to here. It might be Yaakov's sons as opposed to his daughters since "the sons of Jacob" could also include his daughters. But it seems more likely that the group of men who were grieved include both Yaakov's sons and their companions. Since they were a pastoral people, it would likely have included men that were called servants (but would probably be closer to what we would call "employees"). One of these men are likely candidates for who brought word of Dinah's defilement. Presumably he would have gone first to Yaakov and, when he didn't respond, hurried to the men in the field.
The author of Genesis did not speculate on who brought the news because it would have been obvious to readers that once the news reached some member of Yaakov's household, it would certainly get to the patriarch very quickly. We might think of how quickly we find out about some tragedy in our own extended families.
Dinah's assaulter is called "Shechem the son of Hamor the Hivite, the prince of the land" and Shechem is said to have approached Hamor asking to marry Dinah before Yaakov heard the news. So while the defiling act might have been unobserved, there's a good chance that Shechem's desire to marry Dinah was very public. We don't know why Dinah came to Hamor's city, but it seems likely the trip into town was taken by several of her father's people. So to get back to Gill's speculation, it could very well be one of his servants who fled the city with the bad news.