How do you discern the difference between subspecies of Ringneck snakes?

[Praireboyfarmer]

I've caught a few ring neck snakes (I'm not keeping them as pets, simply identifying them, taking photographs and then releasing them where I found them.) and it's obviously a ring neck snake but I'm not sure which subspecies it is. So how do you discern the difference between them?

I found it in Central Oklahoma.