We can identify the mirrors by forming image of an object by those mirrors

I) if the image formed of the object by the mirror is virtual, erect and of same size as object then the mirror is plane mirror.

II) if the image formed of the object by the mirror is virtual, erect and of small size than object then it is a convex mirror

III) when the object is at distance and image formed is real and inverted and if object is very close to lens, the image formed is virtual and erect then the mirror is concave mirror.