An easy way to check the straightness of a piece of aluminum stock is to lay it on the floor and when the clerk isn't looking, use a pencil to trace a line on the floor. Flip the barstock over (edge to edge not end to end) and bring it to the line from the other side. If the bar is staight, then it will match, otherwise any error in straightness will be doubled and easily seen. You can also hold two pieces of barstock up edge to edge and compare the straightness against each other. Then flip one over and retest it to verify that it is straight.