I just re-watched the first episode.
Walt sits in on a meth lab raid with Hank, after seeing how much money meth dealers make from a bust that was on the news, also with Hank.
During the bust, they catch the partner as well as all the cooking supplies and equipment. Jessy escapes because he's banging the chick across the street.
Walt sees that it's Jessy so heads over to his house. He know the address because he still has Jessy's info from the school records.
Anyway, he makes the comment of "You know the business, I know the chemistry."
The other thing is, 2 years have passed over the series, not just 1. He turns 50 on the 1st episode, and he makes his own 52 at the diner in the first episode of season 5. So yeah, 2 years.