Complete feed
Complete feed is a compound feed which, by reason of its composition, is sufficient for a daily ration. It is defined in Article 3(2)(i) of Regulation (EC) No 767/2009.
Is it the animal's whole diet?
That is its intended use: a complete feed is meant to be the animal's sole diet, while a complementary feed has to be combined with other feed.
Where do you check it?
On the label, in the type-of-feed designation, which is mandatory (Article 15(a) of Regulation 767/2009).
Sources
Guide: How to read a dog and cat food label: a guide to Regulation (EC) No 767/2009
Educational information about the rules, not veterinary advice.