You can use the Measure/Ruler tool to determine how much you need to rotate an image to straighten it. To do so, select the Measure/Ruler tool from the Eyedropper tool’s flyout menu on the Tools panel. Next, drag a measure line along an edge of the image you want to straighten, starting at the lowest point. If you have the Info panel open, you can see your angle setting there. However, if you immediately choose Image > Rotate Canvas > Arbitrary, the angle between the two points you measured appears there automatically. Click OK and your image is straightened according to that angle.