There is no conclusive evidence now, just some subjective information that needs further study. I'm not sure I'd do anything now if your dog is doing well on the diet now being fed. If you are choosing a new diet, maybe avoid legumes and potato without specific reason to feed them (like allergy).
These ingredients became popular when the "grain free" fad became popular. Perhaps unintended consequences.
As far as DCM, it presents like congestive heart failure. The diagnosis isn't too big a challenge, but does require ultrasound (echocardiogram) for diagnosis.