How to describe them so the reader knows that the female character is beautiful, other than just "she was beautiful."
I’m not a fan of the other suggestions. Sorry.
And before anybody says it, do *not* have her look in a mirror, describing what she sees to the reader. Cliche.
What I strongly prefer is a brief description ("tall enough to justify her taste for expensive clothing usually seen on runways in Paris or Milan" or "darkly voluptuous and earthy as the olives from her native Greece." No more than that. No description of her hair, face shape, eyes, mouth, figure, grooming, or clothing choices.
Instead, use the reactions of other characters, large and small. When she enters a busy office or restaurant, is there a moment of pause while everybody looks at her, then conversations resume? Does she get offers of service in stores other women don’t? Are the girls she works with jealous, or do they ask for beauty tips? Do people give her a seat on the bus or subway in order to get a chance to talk to her, or just to see her smile? Does she use her looks to take unfair advantage?
In other words, don’t tell me she’s beautiful. Show me.