Most deterrents are made with Grapefruit or some other citrus extract, as dogs generally don't like the smell or bitter taste! If you're going to try a home remedy, I would try something along those lines.

First off, your dog can have roundworms and have no obvious symptoms, so you should de-worm regularly. With that said,the easiest to spot are excessive bum-dragging or licking, small white-ish flecks around your dogs rectum (these are eggs), or even visible worms in stool (they look like spaghetti).