You are supposed to lose mucous plug before you deliver, this is normal. Go to the hospital when your water breaks or you have contractions that are not relieved by resting a drinking glass of water or get closer together and harder but are consistant.
How do you know you're close to delivery? How far apart are your contractions as well. Losing your plug isn't an indicator of labour only because you can lose pieces of it and depending on how far you are it can also regenerate. You can also lose it for weeks before delivery. If you're contracting every 3-5 minutes or less and they're at least 30 seconds or longer you can call L&D and see if they want you to come in. You should also head in if you're water has broke or you're having any bloody show.