When your on the defensive in a thread about your type.
That's kind of an unfalsifiable claim, isn't it? Somebody accuses INTJs of lacking a sense of humor, an INTJ responds that he doesn't believe that's the case, and now he's on the defensive? His very denial is evidence of guilt?
Hmm, where have we heard THIS specious assumption before?
Personally, the INTJs in my life tend to be more prone to/appreciative of humor than INTPs--it's just always got to be high-brow or otherwise clever, or it's not funny to them. This sometimes results in some pretty esoteric shit, but when you really pay attention, it usually is very clever in some weird perspective-shifting Ni way--whether or not it's funny or even makes sense to anyone else isn't the point for them, and that's a really refreshing perspective to me.
NTP humor in general seems more dependent upon positive audience response--that's the Ne for you. INTJs will be quiet for long stretches and then crack the occasional brilliant joke, like...one INTJ friend of mine is incredibly introverted and typically quite serious. A mutual acquaintance of ours got a tattoo on her back that said, "Reject Mediocrity!" and INTJ friend says, "Heh yeah, you take off her shirt, read that, and throw her on the floor." He then smiled, clearly amused with himself while his mind inevitably went on to some other task. NTJ self-confidence in areas like this is really enviable--they simply
don't care if anyone else gets it, because their own inner revelation is enough.