Think he's The One? See if he feels the same way about you. For every 'yes' you answer, there's more of a chance he's envisioning a future with you.
When he has a problem or dilemma he asks you for advice.
You're around his closest friends enough that they're becoming good friends of yours too.
He's started leaving more than just a toothbrush and a spare pair of boxers at your place.
He's increasingly open about feelings unrelated to you, like work worries or family issues.
You see him making an effort to bond with your friends - long after he's already gotten on their good side.