How can you tell if a girl is fit?

Answer 1

You can tell if people are fit by the way they look and act. A fit person is neither too fat nor too thin, and you can see some muscle under their clothing (but not overly-developed like some body-builders!). They can run and exercise without getting too out of breath, and they are usually more confident than someone who is not fit, because they know they are dedicated and capable people.

Answer 2

She goes to the gym a lot.

More seriously -

Beauty is in the eye of the beholder. Everybody has different opinions.