It's really hard to tell. I had one female (who I knew was pregnant) and if I didn't know she was pregnant I wouldn't have been able to tell. She didn't get any bigger, hardly had a gravid spot, and one day just gave birth to about 10 babies. If you got her from a pet store (even if she wasn't in a tank with male guppies) I would say its probably safe to assume she is pregnant.
They are all such a joy. I also have a red blond, and another that I'm not sure of. Its mostly a greenish sheen with a yellow tail and some orange. The blue one and the black one are the most friendly. No fear at all! I never knew I'd enjoy guppies so much!
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