Just set up the boundaries of how much play is too much... if she's not teasing you... then she'll realize that she's inadvertently being cruel and will respect these boundaries.
If she is teasing you, then she will ignore or slowly press the issue with these boundaries for her own amusement.
That would be the quickest way to tell if she is being cruel or not...
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen