It's pretty safe to assume that all female livebearers are pregnant, all the time, unless they're very, very sick or old.
When she's within a day or two of giving birth, she'll what's called 'square off'; that is her belly will develop a distinct boxy shape.
Female livebearers don't need any special care; just good food and clean water. I have to say your tank looks very bare. Not only are bare tanks stressful for adult fish, if you want some fry to survive you'll need some places for them to hide away from the adults, who will eat them. Fine leaved, live plants are best.