The friend who tells you about the spinach in your teeth is the one who’ll tell you the bigger stuff too.

A small awkward correction is a reliable indicator. Somebody willing to risk one uncomfortable second to save you an hour of embarrassment is showing you they will also say the harder things: that you were out of line, that the plan is bad, that something is wrong. Most people would rather protect your comfort than your interests.

Work out which of your friends does this and keep them close, particularly when they say something you did not want to hear. Then do it back, which is the only way the arrangement survives. Start small, with the thing on their shirt, and be kind about it.