If you don't know, or aren't sure, watch the female closely.  Just before she delivers, her gravid spot should be quite dark, and you may actually be able to see the fry inside using a magnifying glass.

If you can see them, put her in the net, or another tank by herself, if you have one set up.
Sometimes I move my fish around to accomodate a favorite female who is due to deliver.

Also, her gravid spot should drop and shift slightly toward her vent.  This will give her a "squared off" appearance, looking from behind.

She will not stop swimming, or do anything unusual before she delivers.

Sometimes when you place a pregnant fish in a breeding net, they stress out. If this happens take her out, and take your chances of the fry being borne in the tank.

Increase your chances of saving the fry by adding more live plants, including some that float.
Some live plants are less expensive than plastic, they are great places for the fry to hide in, they improve your biofilter if kept neat, and add extra oxygen to the water.

Always rinse live plants very thoroughly before adding to your tank.

I hope this helps you.
Might give you some things to do to keep busy while you wait.
That can be tough for a guppy addict.
Let us know how you make out in a response or a diary entry. OK?