Place your level on top of the brick structure. Determine whether it is straight with the indicators on your level. If the structure is long, place your level on several parts of the structure and check the level indicators.
Split the brick structure vertically and horizontally into four or six segments with your eye. Splitting up the structure this way can help you pinpoint any problem spots since only a portion of the brick work may not be straight. Place your level against the brick in one of the segments so that the bottom edge sits against the top edge of several bricks. Using the indicators on the level, determine whether the joints are straight in each of the segments. Do the same with the each of the other segments.
Go to the edge of the brick work and flip the level vertically. Lay the level against the edge and use the indicators to determine whether the structure is straight. Make sure you do not rest the level on the ground because the ground may not be straight. Continue this process along all of the vertical edges of the brick work.