How friendly is he toward you? Does he make small talk with you every chance he gets? Does he go out of his way to make small talk with you? For example, does he walk all the way around the building past your office on the way to his, even though it's double the distance ordinarily necessary for him to walk, so he can say something to you? Does he look for reasons to call you at home after hours? Has he hinted about the two of you having lunch or coffee sometime? These and other similar behaviors would be signs of interest.
