The INTP, a rare and reclusive specimen, is almost never present in the wild of his own volition, since he prefers to remain in his dwelling lurking various forums in order to amass and categorize vast quantities of only tenuously-relevant information.
If one should ever gain access to the computer of an INTP - often his most sacred object (thought when stolen away quietly its absence might go unnoticed for a great deal of time) - a few distinctive features will stand out. First, look at his browser. If there are five different typology or MBTI-type-related forums open concurrently, or if the desktop looks like it's being organized by three different systems at once, without any of the three being complete, then chances are your specimen is an INTP.
In writing, the defining feature of the INTP - aside from his often-cruel wit, will be his vicious pedantry. Expect constructions like "to who it was given," or "what helped me was she being there," to bring on comparisons of the user's intellect to that of a primate or an inanimate object. Spelling errors are seen as sins meriting flagellation. The use of 'u' to stand for 'you' or other such instances of greatly altering the English language will practically guarantee the cessation of all communication with the user by the INTP; however, often-byzantine abbreviations such as 'tbqf' to stand for 'to be quite frank' are used frequently, and the INTP will express genuine incredulity whenever the other party is not able to instantly decipher any new abbreviation the INTP has used. In addition, in personal communication the INTP is not necessarily insistent on proper capitalization and punctuation.
In the wild, should one happen across an individual wandering forlornly and aimlessly looking alternately at the ground and at the clouds as well as avoiding eye-contact with any passers-by, then one might have spotted an INTP on a rare fresh air excursion. Additional indicators of INTPdom might include sudden changes in direction, a sporadic avoidance of various ground elements such as cracks or tiles of a specific colour, occasional stops to stare blankly at nothing with an emotionless face. The speed with which the INTP walks is almost invariably inversely proportional to his knowledge of his destination.
A very good indicator of INTPdom is smiling or smirking without any obvious cause. Also indicative are sudden stops in motion accompanied by a rapid movement of the head, with eyes searching all around, as though the INTP were sniffing out the scent of an idea hiding behind a metaphorical bush or tree, his Ti thirsting to disembowel it.
Owing to the fact that most INTPs will spend their free time writing unnecessarily complicated, tangentially relevant responses to various threads on various forums, the INTP could be perhaps the rarest type to spot in public.
[The above rant was written based on my experiences. The description of the INTP's going for a walk is 100% true to my life.]