I hate to say you will know, but you will. For me my contractions got close together and really intense. So much so that I couldn't talk during my contractions and had to force myself to breathe. Timing your contractions is really the key, if you get them 3-5 mins apart for at least an hour and they are really intense then it might be baby time. It typically doesn't happen right after you use your plug. I went into labor weeks after I lost my plug.
From what I understood if your mucus plug falls out then call the doc or your pain gets more intense. I read somewhere that a lot of women feel cramps for weeks leading up to labor and when contractions start at first you will be able to continue with normal activities, but once they get closer together and last longer then head out to the doc.