terriers

Question
how can you tell differ from rat terrier to fox terrier

Answer
Ron - I am assuming you are talking about smooth fox terriers and there are definite requirements for both. You can go to the AKC website or to the club for the breed and they will have all the standards listed. Personally, I see a huge difference. Size, heads, ears, angulation, are all very different.
But check the websites and then go to a couple of dog shows and look at the breeds.
Have fun!