Swordtails are pregnant essentially all the time.
If you have males and females, the females are pregnant. They can have anywhere from 5-100+ fry, depending on the female. 15-50ish is probably average.
If you look on your females, you'll notice they have a gravid spot. That's the most commonly used tool in predicting pregnancy. You can find this on the stomach where the tail meets the body. The fry develop there, so the darker it is, the more developed the fry will be, which also means the mothers are further along in pregnancy. Females always have a gravid spot, since they store sperm and eggs and such there too, so don't be confused if they have it after they give birth. You'll notice it will be lighter though. Near the end of pregnancy, if you look closely, you may be able to see the fry's eyes in the gravid spot too.
The mothers will be fat too, of course. That's the easiest way to see if they are pregnant. Some get huge and some don't. You'll have to see what your's do and then you'll know for next time. But honestly, with males and females in the same tank...the females are pregnant. Good luck!