If I liked my vet, i would never change vets based on the food they sell. Most vets know very little about food....but the good ones do know about tapeworms.
Did you describe other symptoms to the vet? (like seeing white tapeworm segments in the poo)
If you did, then the vet may have diagnosed the condition based on additional info you may not have added to your post.
From my understanding...it's easy to see tapeworm segments in the poo. But not every poo has these segments.
I do not think a vet can feel tapeworms in a stomach.....but how the stomach feels along with what you may have told your vet might have suggested tapeworms.
Here's an article that may help