To perform visual comparison of two DWG drawings you can use command DwgCompare.
Open the first (original) DWG file in OdaMfcApp and vectorize it. Execute DwgCompare command by simply typing it or selecting "Edit" -> "Registered Commands" -> "Dwg Compare" -> "DwgCompare". Following that, the dialog window will open. Choose a DWG file that you want to compare with your first one and click "Open". Now the comparison will be performed and you will be able to see the results in the same view window you had open.
Entities that have been changed, removed or simply don't exist in the file you are comparing with the original will be colored in red. Entities that have been added or changed relative to the original file will be colored in green.
Now you can visually compare the two drawings. Once you entered the DwgCompare command, you will be staying in the context of this command. To go back to viewing the original file you need to execute command DwgCompareExit by typing it or selecting "Edit" -> "Registered Commands" -> "Dwg Compare" -> "DwgCompareExit". After that you can continue with your work on the drawing.