I have a show strain English Springer Spaniel ,she is just over a year old but I have only had her for 3 months. I was interested in showing her ( something I have not done before ) but is it too late as she has never been to ringcraft classes or is it still possible to with older dogs. Also, how do you know if they are good enough to be a show dog?

It is perfectly possible to show adults.  Initially until they and you gain expereince and confidenc in the ring it may affect your placings by a place or two, but after that you will soon be on a level footing.

I didn't start showing my oldest Elkhound bitch until she was 22months old, and a dog that I bred who is now a Champion wasn't shown until around two years old either.

I wopuld take her back to her breedr for an assessment of her quality (assuming the breeder shows) to see whether they think she is worth showing, though you could just enter the shows and see how she fares. 

I would suggest at least a few ringcraft classes for you both, and also fin out how they should be groomed and triummed for the ring (again your breeder should be the best person not a local pet groomer as few know the show look).

If you don't know the breeder, as I gather she is a rescue dog? go along to a show with her either entered normally or not for competition, or take some good photos of her head and of her standing, and people ought to give you a reasonably honest opinion. Unless of course she's so fantastic that they don't want her in the ring beating their dogs!!!