Two shapes which are equal in shape and size are called congruent. Although you should be able to see if shapes are congruent, the most complicated shape to understand is the triangle. There are several rules to spotting congruent triangles:
All three sides of the triangle are equal to all three sides of the other triangle.
Two of the angles and a side of a triangle are equal to the corresponding two angles and the side of the other triangle.
An angle between two sides of the triangle is equal to the same angle and the same sides of the other triangle.
Two right angled triangles which have the same hypotenuse and an equal corresponding side.
Also if all 3 angles are equal on each triangle then the triangles must be similar!