Marti likes to preview blocks by reflecting a small portion in a pair of mirrors; the multiple reflections create the illusion of an entire block. In the diagram, a portion of the blocks is reflected, showing what the whole block would look like with those colors in those positions. You can also use the mirrors on a piece of fabric to see what specific printed patterns will look like repeated in a block.
Cut one triangle or enough pieces for a portion of a block. Position the mirrors and place a piece of tape across the top to hold the angle. Preview different color combinations and prints.