I found a company that you can post a sample of your dogs poo to and they look at it under a microscope and email you to say if your dog has worms.
I think it'll be interesting to see what they say. I've wormed the dog regular with Drontol so thought he was fine. Then used some advocate for fleas and a week later there were some roundworms in his poo. This was the other day so I'm a bit freaked out. The vet said that the worms must be dead if they're coming out, I'm not sure though. When he first pooed there were no worms, ten mins later I could see worms so they must have been moving.
The company sent a little kit out. With a tiny box to put the poo in. It was all a bit gross. I had gloves on but managed to get poo on the outside of the box so I feel sorry for the person who opens the envelope. Though hopefully they have gloves on as well. 