How does Telemachus know Odysseus was his father?

Telemachus' mother was with him for the first 20 years of his life without his father. She might have told him.

Odysseus was very popular in his home, the city of Ithaca. The town's people would have told him.

Telemachus was once a toddler. We all know how questionable little toddlers are! he would have defiantly asked as soon as he could talk.