Honestly, an equally intelligent ISTJ and INTJ with well-developed Te would be pretty tough to tell apart, particularly in the workplace.
...in personal relationships, that one-letter difference causes us to drive each other insane. Our actions can be similar, but our motivations are quite different.
If the person you're attempting to type is looking to be a pillar of strength to the workplace/community/etc., he's most likely an ISTJ. If he's working to realize some secret plan which he refuses to tell anybody about, you've likely found yourself an INTJ. (This can be misleading, however; if the INTJ doesn't trust you, he'll swear up and down that he does everything he does for the sake of being a pillar of strength to the workplace/community/etc. Business-minded INTJs, in particular, are perfectly willing to blend.)