Han Sen recognized his father's handwriting due to the vast number of family documents and possessions he had collected and perused over the years.
The letter he had received had been written and sent by his father, there was no room for doubt. It was easy to fake someone's handwriting, but the content contained information only he and his father would know about.
And near the end of the letter, he read something that was most shocking.
When Blind Man provided Han Sen the cauldron, there was a pill inside. He claimed it was a gift from his father, and his consumption of it was of absolute importance. He was told to eat it before becoming a demi-god, due to its ability to aid him in unlocking something.
This gene lock was not associated with the average ten gene locks, and he had been told it'd be of great benefit for when he finally did become a demi-god.