How can you determine your dog's food preferences reliably?
 
One method is to provide a different type of food in his bowl each day and observe how much he consumes.

For a more precise approach, you can weigh the bowl before and after feeding to calculate the amount eaten. 

However, this method may have limitations. For instance, what if your dog consistently cleans the bowl regardless of the food offered?

Or what if his appetite varies from day to day? Additionally, one food might be more filling than the other, skewing the results.
 
Another approach is to offer two bowls of food side by side and let your dog choose. Pay attention to which bowl he eats more from. Yet, this method also has potential pitfalls.

What if your dog licks both bowls clean? What if he simply eats from the bowl he reaches first?

Or what if he shows a preference for eating from only the right-hand or left-hand bowl?