If you have a form that fits that description, it is also all of the following: Figure of a plane figure with four straight edges and four right angles. A box. There are many ways to identify if something is a rectangle. You can use your mind or you can use software. There are even websites that will identify shapes for you. Here are the most common methods: 1 Use your Mind. Think about what a rectangle looks like and then draw some lines on a piece of paper. Repeat this process with other objects (such as squares or circles) and see which one feels most familiar. 2 Calculate Dimensions. The dimensions of a rectangle are its length and width. To find the dimensions of a shape, simply divide the length by the width and multiply it by itself. So, for example, if the length of a rectangle is 8 inches and its width is 4 inches, its dimension score would be 2 times 2 or 4. Squares and rectangles with same dimension score are considered equal in size. 3 Measure from Point-to-Point. One way to know if you have found the right thing is to check whether the measurements you get are the same.