when guppies are pregnant, you will see black spots and she will get extremly fat compare to normal size...and will take a bit
then you want to put her in breeding trap or breeding tank and do a bit of water change and it will kick in
oh...then you should be just fine..just make sure to have like sponge filter or something..and i'd put em in breeding trap once the spot get super dark..or if you see female guppy lay down on floor a lot or not eat much..that should be sign of geting close to it..and here is pic of my tank