She will be pregnant already; most livebearers are pregnant, all the time.
When she's a day or two away from giving birth, her belly, especially the the front, will develop distinct 'corners'. This is called 'squaring off' and is the best way of telling how close she is.
One she's had that brood, she'll give birth again in 28-32 days; on average 30 days, but depends on temperature.