Well, look at it. It should have clear eyes. About as wide as it is long, but this varies as they inflate and deflate with air and water.

They are very sedentary frogs. But they have a strong 'righting' reflex.

Flip the frog onto it's back. It should quickly and vigorously right itself. Sometimes they can't get a grip and may struggle upside down. If so, help it out before too long.

As long as it vigorously tries to right itself, it's got energy and it's doing a typical behavior, so that is one measure of health.