I always do this, i'll suddenly think about taking a test again because i'll think i've suddenley changed and doubt that I answered the questions truthfully so i'll answer everything as truthfully as possible and i'll always turn out more infp than before. I think reading these forums has made me even more of an infp than I was to start with.
I've taken the test the tests a few times myself. I only score INFP, though I wonder whether part of me is worried about scoring anything else (I love being one of you guys) so I am subconsciously taking choosing the answers to the test which would confirm me as INFP again. I may be wrong but it's a consideration of mine.
Hanging around in these places teaches you a lot about your type as well. Other than that I suggest studying cognitive functions, it takes some research but it becomes quite easy eventually in my opinion telling apart a function you have which you don't necessarily understand fully and a function you have no clue about which you never used.