No problem.
I think an easy way to tell would be to spot out the Ni and Si differences.
From my personal observations:
If he's INFp, he'll probably seem like he's in a dreamy, high state more often than not. They usually have a distant, starry, cloudy look in their eyes, and can seem to be drifting off in their own world, kind of torn off from reality. From a distance they can appear to be silly, deep, mysterious, allusive people who are somewhat difficult to pin down.
I've noticed that ISFp's tend to be a lot more openly friendly and loving, though still introverted. They usually have an earthier feel about them, and they seem a lot more aware of their surroundings. It's easy to see them as warm, friendly, caring natured people, who have a childlike silliness about them.