The easiest way of distinguishing between a convex, concave and plane mirror is to see how the characteristics of the image change when the position of the object is changed.
In a plane mirror, the image is upright and the size is always equal to that of the object, irrespective of where the object is kept. If the object is moved closer or farther away from the mirror there is no change in the characteristics of the image.
In the case of a convex mirror, the image is virtual, upright and its size is diminished. There is no change in these characteristics irrespective of whether the object is moved closer to the mirror or away from it.
The image created by a concave mirror displays changing characteristics as it is brought closer to the mirror. At a large distance it is inverted and as it is brought closer it changes direction and becomes upright.