He doesn't know.
We see the Mark from his perspective:
At once, Harry's scar felt as though it had split open again. His true surroundings vanished: He was Voldemort, and the skeletal wizard before him was laughing toothlessly at him; he was enraged at the summons he felt - he had warned them, he had told them to summon him for nothing less than Potter. If they were mistaken...
Harry Potter and the Deathly Hallows, chapter 23
We see that he doesn't know what he was summoned for. It's not clear if he knows who, but we see that he knows where to go - it summons him there.