Trust me, you will know.
About 2-3 weeks before your baby is ready to come out, you will likely lose the mucous plug that is blocking your cervix. It will look like bloody snot. Some people never lose theirs....but not to worry.
Another sign to look for 2-3 weeks prior to the baby's arrival is colustrum....a thick milk seeping out of your nipples as your milk glands begin to prepare for the baby's arrival. Again, some people don't notice this. But don't worry...you will get milk shortly after the baby's arrival.
You will also begin to notice 2-3 weeks before that your "baby bump" is lower than before. This means that the baby is lowering itself further down into your pelvic area in preparation for birth.
Mentally, there is a period about 3 weeks before birth that is called "nesting." This is when you begin to clean and organize everything in your home in preparation for your baby, and you won't likely even notice how anal you have become about having everything clean, tidy, and in it's place. It is referred to as nesting because it's sort of likened to a bird making a nest in order to lay it's eggs. LOL.
Also, look for contractions. Every little pain you feel might make you think it's a contraction, but TRUST ME...you will know when you are experiencing one! It is an intense feeling of tightness like the worst menstrual cramps you could ever imagine.
When you are in active labor you will know because you will feel an overal sense of not feeling like yourself. You may become quite nauseous, hot, restless, and anxious. Your contractions will be growing in intensity and in frequency. Your water may or may not break, but you will likely feel cervical pains as your cervix begins to efface, or thin out, and dilate.
At any rate, you will know when it's time. It is not easy to miss going into labor at all!
Keep in mind, too, that some women may not go into labor at all and have to be induced at the hospital. Don't feel bad if this happens to you. You will still get to experience everything that someone who goes into natural labor experiences, only at a higher intensity level. So you will not miss out on any of the excitement.
I am not a doctor, but I have a 5 year old son and I am about to have another son in the next few weeks. I watch all of the baby shows on TLC and do a lot of research so I am always prepared for whatever might happen.
Don't worry and don't stress. Stress may actually delay labor further.
Good luck to you, and I hope all goes well!