Well, first I weigh all of my chins weekly. Weight gain is a good indicator of pregnancy, and loss may signal illness, so it's a good idea to keep tabs on it anyway. But most females also show definite signs prior to delivery. Besides weight gain you can tell how far along they are by the length of their nipples, which are located in a straight line on their belly beginning in their "armpits". By the end of pregnancy the nipples will usually be as long as the fur, and many times the fur will actually fall away from the area. Most females will begin sleeping A LOT and sleeping on their sides. Generally once I see a female sleeping on her side a lot I move her to a maternity cage and then begin checking her every few hours or so. I always separate my pairs during the last stages of pregnancy and for a few days after delivery to prevent breedbacks. I also stop dust baths at this time as well.
Most females will also consume a MASSIVE amount of water just prior to delivery, so it's a good idea to keep tabs on the water level too. I guess the point is, there are a lot of ways they let us know what is going on, without having to witness the mating or to find the plug. Partially, I think, it is also a matter of experience. It's easier for someone who has seen it a lot to recognize the signs than for a newbie. I know I hover over my females constantly near the end of a pregnancy, and overanalyze every little thing sometimes. Drives my husband (and probably the poor chin) crazy! But I can feel myself relaxing and becoming more comfortable with every birth, so I am thinking that time and experience are the keys.