How do you know if a girl has a BBL?

I'm curious, how exactly does one determine whether a girl has undergone a Brazilian Butt Lift (BBL) surgery or not? Is there a tell-tale sign that's easy to spot? Or does it require a more nuanced approach, perhaps by observing her physical movements or the overall balance of her body? I'm genuinely interested in learning more about this topic, as it seems to be a popular subject of discussion in many circles.