Colloquial sentence endings like "is all" and "is how"

I'm writing dialogue for a short story and I want to 'transcribe' the characters' colloquialisms in a way that best adheres to the rules of written English (I'm an ESL speaker). My protag ends some of her sentences with expressions like "is all" and "is how" (e.g. "Just asking is all"). My questions is, Is that type of expression supposed to have a comma beforehand? Or would that be a comma splice? Example from the short story itself: "How did you know I was over by the fountain?" "Sister Amy told me [,] is how." The descriptivist and prescriptivist parts of me disagree on the matter. Thanks in advance, folks. :)

You would not need to add a comma to "is all," however, you would likely need to add a comma if the phrase were "that's all."