How did medieval European military, which did not have standardised uniform, told ally to enemy? Especially for men-at-arms who did not have any identifiable element in the clothing at all?

More can always be said on the matter of medieval combat identification; for the meantime, as this crops up fairly frequently, I commend to your attention these following previous posts:

u/WARitter goes into the various means of identification combatants had then;
u/Goiyon looks at the same, and also notes the dimensions of combat then;
but this, clearly the best answer, is one straight from His Majesty u/Henry_V_Rex.