The first question to ask if you think your fish are pregnant is thus:
Are they livebearers?
If the answer is yes then yes, they are pregnant.
The only way to ensure that your livebearers aren't pregnant is to quarantine the females by themselves for two months. After that time they will have likely had all the fry they are going to from the last male they mated with. Alternately you can separate out the females after birth and isolate them.
Of course this goes out the window if you have males in the tank. If there are you can be sure the females are in some stage of pregnancy almost at all times.