Since the two diagonals bisect each other, you have created two sets of equal lines. If that is the case, then you could prove the equality of the triangles 2 different ways.
1) The angles at the center are vertically angles which makes them equal. Since they are equal, the triangles are congruent by SAS. Notice that the angle is between the 2 equal sides.
2) Since CD and AB are marked as being equal, the triangles are congurent by SSS.
