You mean pregnant or right before she is going to drop the fry? Cause if its the first one, the gravid spot is an indicator. Its a dark spot right before the vent. If it the latter, its hard to tell. Usually, if you look at the swordtail from the back and it looks squarish....thats when she is REALLY close. At that time I usually put her in a seperate birthing tank and do a water change with hard alkaline water...and she usually drops that day or the next.
!
