When are you due? Sometimes you might notice the contractions as more of a backache that comes and goes, and starts to get a pattern to it.
Losing the mucus plug can be a possible sign that your body is preparing, but it can also regenerate and you could go own for weeks still.
Some women will start to feel a bit ill...have loose bowel movements, maybe vomit...sort of like having a stomach bug either right before labour starts or in very early labour.
Water leaking, but not actually breaking...might feel like you need to use the toilet, but still feel some fluid leaking every several steps after you are done. Could just be more discharge at the end of pregnancy that is common, could be some urine leakage, but it were to continue, would be wise to have it checked.
Light bleeding or spotting....can be normal if you had recent intercourse or a pelvic exam, but if neither of those were in the last couple of days, it can be from the cervix starting to thin out.
In most cases, it will become fairly obvious that something is going on, and of course, when in doubt call your midwife.