Cry from a Far Planet by Tom Godwin, Science Fiction, Adventure

A smile of friendship is a baring of the teeth. So is a snarl of menace.

It can be fatal to mistake the latter for the former. Harm an alien being only under circumstances of self-defense.

TRUST NO ALIEN BEING UNDER ANY CIRCUMSTANCES.

--From Exploration Ship's Handbook

The problem of separating the friends from the enemies was a major one in the conquest of space, as many a dead spacer could have testified. A tough job when you could see an alien and judge appearances; far tougher when they were only whispers on the wind.