In 45:27 the pasuk tells us, "וירא את העגלות אשר שלח יוסף" - Yaakov saw the wagons that Yosef had sent for him. Rashi explains that Yosef sent the wagons as a sign to Yaakov. They had been learning together the topic of Eglah Arufa when the incident in which Yosef disappeared had occurred. The wagons, agalah in Hebrew, was a play on words to show Yaakov that his son remained faithful and learned.
But I think Yosef did not send the wagons. And if he did, they were only on the command of Pharoah, so he could not have sent them as a sign.
Why do I say that? Because in 45:19 we see very clearly Pharoah telling the brothers of Yosef to take wagons and bring back their father to Egypt. Furthermore, in 45:21 it says Yosef gave them wagons on Pharoahs word. Furthermore, in 46:5 it says that the children brought Yaakov from Beer Sheva in the wagons that Pharoah had sent to carry him.
So how could Yaakov have understood anything from the wagons?
It is possible that once wagons were being sent on Pharoahs command that Yosef made some alteration to the wagons or some sign on the wagons to indicate the message to Yaakov, but that is not clear from the Midrash. The Midrash quoted by Rashi seems to say that simply by the fact of the wagons being sent the message was conveyed. The Midrash is based on the fact that the passuk in 45:27 says, "The wagons that Yosef sent" rather than associating them with Pharoah as it does elsewhere (in the other pessukim I quoted), yet I still find it difficult to accept that answer. Yaakov would not have known who sent the wagons and could only have assumed they came from Pharoah.