Save your design, and then make a change and then go to File > Save As > Save As New Version. This will append a number to the end of the new save file and basically create a second copy of the design. Now go to the Detail tab and on the right hand side, in the 3D Markup section, click on “New Slide”. Now go to the Insert tab and insert the previous version of the same file. Go back to the Detail tab and place a dimension on the change you just made as you normally would and then click on the “Original Dimension Values” button in the Detail tab in the 3D Markup section. You will see an “Is” dimension and a “Was” dimension, where you can visually see the difference between the two pieces of geometry. You can then move the components around if you want it to be side by side instead of on top of each other.