I have 2 guppies and 1 swordtail that all have dark gravid spots. The swordtail looks a bit plumper than the male but the guppies not that much. How could i tell if they were pregnant or not?
With Livebearers, if they are female, there is a very good chance they are pregnant. It sounds like all of your fish are pregnant, especially if the gravid spot is quite dark. They might be a ways away from having fry if they are still looking fairly thin, but I'd look for some fry within the next month.
As ChesterScot said, there is a good thread with lots of pictures started by 5teady that will help you learn to recognize a pregnant fish. I have a link to it in my signature area marked as Ready to Pop.