I wish I knew. I have trouble with this with even my closest friends. I have a difficult time saying "you know, this girl probably isn't right for you" which after they break up they say that they wish I'd have said something.

The only thing I can think of is to make a concerted effort to be honest. Not too honest. I have noticed that if you're too honest it comes off as rude and abrasive even if it's true. Hopefully that will encourage my friends to be honest with me which is the kind of honesty I value the most from all the people I know in life.