1.the hold just under its anus open became more bigger or obvious.The place where eventually the fry will come out.
2.The fish will tend to hide in the area that have less fish such as cornered of the tank.
3.Always go near the filter ,aquatic plant,wood.stone or rock to get shelter for its fry.
4.Some may also not even eat before giving and during birth process.
Beware the livebearer such as swordtail may even eat or consume their young.
Try to separate the mother and fry using a breeding tank/trap.
Tips:
Feed the female with mix diet of food like food pellet,worm and supplement with green food and vitamins to get more yield or fry.
Hope my advise will help you on this matter and good luck on your breeding project.