how do you tell the difference between a real pregnancy and a false one? my bitch has not been mated but she is showing signs of pregnancy and im worrying that a dog might have mated with her when she is in the garden, although I kept a very strict eye on her when she was in season. Im 99% sure she is NOT pregnant but it would be good to have the little doubt removed, is there any way to know for sure?

Only time will tell for sure.  If she goes more than 10 weeks from the time she might have been mated then she isn't preganant.

Try going to your vet?