You can usually tell whether someone is committed to change. Seeking counseling or therapy and trying hard to work on yourself is one way to know. It's very rare for a person to do that, though. Most people will just tell you they will change with no actual plan in place, and these people will very likely not change.

If someone cheated on me and told me it was the last time and they promise they will change, I would not take them back. If they told me what they would do in order to change (dig into their past, figure out why they did it, have a conversation with me about how it affected me, look for relevant patterns in their childhood, etc) I might do it, depending on the level of progress and commitment