First off in my experience, INTPS and INFPs, the most individualistic of all teh types, have the hardest time believing that any typing scheme could possibly describe their uniqueness
This may seem simplistic, but I find that the key words for each preference dichotomy are often key to helping people determine which describes them. For T and F, it's decisions--where do their first thoughts go in decisions. Granted T principles and F values are tough to distinguish between but there's an objectivity to INTPs that is recognizable. The second piece that really helps is getting at the inferior--which actually trips them up.
So...if someone can't decide between INFP and INTP I'd start with what concerns them most when they disagree with decisions. Is it the impact on the people involved or the lack of logic/fairness in the process? What's their first inclination when someone asks for an exception--to stick to the rules (T) or make an exception (F) (this works great once someone has children; every child knows who the Feeling parent is...). As a child were they seen as overly sensitive (F) or detached, observant (T). I have a book called
LifeTypes by Hirsh and Kummerow that has a chapter on each type that begins with about 3 paragraphs on the childhood of each type--stories told by adults thinking back who know their type. I have people read the INFP and INTP paragraphs and 9 times out of 10 they recognize themselves.
In your environment, making sure the client understands the deep value of both the persuasive style of INFP argument and the logical style of INTP argument. Sometimes they can start to decipher which is natural and which is learned. For me, INFJ, those first logical English lit papers were painful--I had to work from a clear outline or I'd lose my line of argument. I wrote my dissertation with a matrix in front of me or I'd have fallen into my persuasive traps (did a few times anyway and got to rewrite...).
If that doesn't help, I go at it backward. Are they more likely to trip up in figuring out how others will react to something (T) or in failing to stay objective about something (F).
Does this help at all? Interpretation is such an art that it's hard to think through when the person isn't interacting with me...