how to know that your friend is true to you?

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..I've recently discovered a theory regarding true friends..later this evening I've come up a few realizations..I was talking to a person for quiet a while...we've been discussing a series of things. At the spare of the moment I decided to go to my room I looked myself in the mirror then I realized that I have a booger in my eye.So would I consider that person a true friend not telling me what was in my eye the entire duration of our heart to heart discussion?... by that simple matter, i believe a true friend, or rather a real friend could easily tell me without even having doubts whether he or she could hurt my feelings or not..