We only know for sure that the Communion of the Saints is for real. We do not know the names of all the saints in history. We do know the identities of those whom the Church has chosen to honor and whose intercession it specifically invokes........whose feast days she celebrates.
But you are correct in saying that there is no way of being absolutely sure if all of the saints names under the "old waY" are infallible. However, one does have enough documented stories of answered prayers, and seemingly miraculous intercessions, to know that some saints are "for real".