How can you tell if someone really love you?

Love is such a fickle beast. Do you mean in the past, at present, or in the future? If xyz happens to them, happens to you, or did or didn't happen to both of you? Are you asking if before or after or during one of you or both of you has an affair? What's love? If that happens did love not exist, exist weakly, or exist strongly? Being in love, having a peak, or valley, or apposing peaks and valley's? I think you can only look back at history and really properly evaluate whether love existed at a point a time, and how strongly. Love is not a guarantee of anything in the future.