For the situation you are describing, the best solution would be "this is a tell-tale sign of psychological illness" (= it is indicative of psychological illness, it tells you that the person is likely to be psychologically ill).
"This is telling of xyz" is used slightly differently. "50% of respondents indicated that they would not talk to a close relative about their issues. This is telling of our attitude as a society towards psychological problems" ( = this tells you something about our attitude). Note: "this is telling" not "a telling" - the latter sounds weird, unless you are talking about a tale, perhaps.
"This tells ..." requires an indirect object like "you, us, ...": This tells you something about the issues he has. This tells you he has issues. Less formal than the rather clinical: this is indicative of his psychological issues or the more neutral this is a tell-tale sign of psychological issues.
Hope I covered all the common options ;-) Update: Haha, and I just read Jimmy's comment and realize I did not. Didn't think of "this tells of" ;-)