God show me how

When you love someone it is our responsibility to let them know when they are doing bad... right?  But what if you really are not sure, and they don't tell you- they refuse to open up to you but you can see that they are doing bad. How do you gently tell them that you are disappointed with their choices, and that they are clearly not in the right path? Where do you cross the fine line between grace and truth? But how am I to be clear that my path is right and theirs is wrong?  I know that Jesus says I have to take the speck out of my eye first before I pay any attention to my sister's speck, SO THAT I am able to help her take it out. But how do I know? How do I say? A letter, a phone call, face to face confrontation.... 

This discipline of accountability is not easy. God be with me....