If the only symptom is gas, I would think that it would not be all that easy to run a test and be able to tell for certain. I'm sure a blood test would show it, but to what degree would that be? And, blood test results can vary. For example, my son's first blood test showed he had a slight allergy to wheat only. The follow-up blood test 2 months later showed he was not allergic to wheat, but that he's allergic to bananas. Go figure. At the end of the day, I think you'd find that regardless of what shows up, you'd end up with having to change foods to see if it helps anyway.
If the gas doesn't go away in a couple of weeks, I'd go back to the Eukanuba. From what I've been through lately myself, I'm realizing that it's tough enough to find a food my dog's digestive tract agrees with, so if your dog was on one that worked, why switch? Eukanuba's a good food. Sure, some people would undoubtedly argue, but it really is. And, there really is a point when too much of a good thing is bad. Like when you ingest Vitamin C. You take too much, and the body then flushes it, ALL of it, and then none of it is retained. I wouldn't doubt it if the same would be true for many nutrients.
My theory is that it's better to be on a decent food where nutrients are absorbed and assimilated by the body, than a so-called "higher quality" food that ends up being passed through largely unabsorbed. I know of so many people who's dogs just can't maintain a good stool on the high end foods because they are seemingly to "rich" in some nutrients. And, interestingly, the owners themselves all independently used the word, "rich". Makes you think.
Since your dog's stool is not affected, it could be just one ingredient that's not liked by it's system. But, I sure would not want a dog with bad gas in my house. Been there, done that!