We look our face in each mirror, in one after the other. First we keep our face quite close to the mirror and then move it slowly away from the mirror.
1) If the image formed is of same size as our face but laterally inverted for all positions, then it is a plane mirror.
2) If the image formed is erect and enlarged initially but gets inverted as the face is moved away, then it is a concave mirror.
3) If the image formed is erect and smaller in size for all positions then it is a convex mirror.