When my females are going to give birth I can usually tell because (aside from looking like they couldn't possibly get any fatter!) they are quite still and tend to hang around near the heater, or at a corner of the tank near the surface or gravel. The gravid spot tends to be dark all the time, due to them always being pregnant! I've never thought mine look particularly box-shaped when ready to drop though a lot of people do say this.
I think your tank is probably overcrowded now, you will need to do frequent water changes and monitor the ammonia, nitrite and nitrate levels carefully. How long has your tank been running now? I only ask because I made this mistake recently and put too many fish in a newly set up tank and the result was not good! Two of my fish died (one instantly, one over a couple of days which was awful to watch), others got poisoned by the toxins in the water and now have red gills and probably long term health problems which I am yet to discover.
:( Besides, bigger tanks are much more fun, and also actually easier to maintain as well!
Also, if female guppies get stressed they can actually abort their own young (I think mine have done this due to the tank disaster and all the moving about I've had to do with the poor things, plus living in a 7 gallon tank for a while with the 3 males has meant they have nowhere to escape to when the males are constantly harrassing them).
Are you planning on moving them to a partition or something before they give birth? I just let mine give birth in the tank then fish out the fry if I want to keep them, and put them in a separate tank or breeding net, I think it's less stressful for the mothers.