Atwood conceals Grace's innocence or guilt by making Grace the key narrator who will speak on behalf of what she knows, leaving us, the readers, to serve as a kind of jury of her peers. It is up to us to decide whether her word is worthy enough to be taken for granted without any doubts. Additionally, Grace's words are always ambivalent, and could always be interpreted either way.
Just because he [Dr. Simon Jordan] pesters me to know everything, is no reason for me to tell him.
Atwood does this for a number of reasons. The novel takes place in the 18th century during a time when women had no power in society. Grace's word as a poor woman, and as the servant of a prominent man, would have amounted to nothing regardless of what her testimony may have been. In a way, it is society that makes Grace the ambiguous character that she is because of the determination of that type of society to classify women as "good or bad" in an unrealistic way.
Another reason is that giving Grace the power of ambiguity, and of keeping a secret, makes the woman who would have been otherwise considered by society a dull, prudish nobody into someone who is capable of eliciting the interest of Dr. Simon Jordan. In many ways, allowing this ambiguity in the storyline is what empowers the character of Grace. In a way, Grace's empowerment is also that of all the other women of her time.
We will never know if Grace is guilty, because it is never stated. However, after putting some cues together, this seems like a possible scenario: it is likely that she has been coerced to accept guilt like she could have been made to do many other things. She has no control, nor say, in any aspect of her life. She is at the bottom of the social ladder, and people who are in that situation are often abused and used as scapegoats. Also, Grace already comes from a troubled past and her behavior in prison shows that she has never expected anything from life but what is given to her. Hence, it is possible that Grace admits guilt because she knows that nobody will do anything to come to the defense of a servant anyway. Why fight it?
Her demeanor in prison, where she is so well-behaved that she earns the privilege of working at the governor's mansion, is very telling. It shows that, as she accepts the fate that comes her way, she also trusts her inner goodness, and it works. She is compensated with a job in prison that is likely much better than what she was doing prior to the conviction. As a natural talker, she may have even enjoyed the attention from Dr. Jordan, an attention that she likely never had before. Hence, whether Grace is a sociopath or an innocent victim, her secret and her ambiguity is what empowers her. She represents every woman who has gone forever unnoticed and suddenly realizes that they, too, have a voice and the choice to use it...or not.