As expected from air-based abilities, it's faint and subtle and really tests Toph's ability to perceive something. She cocks her head slightly to listen more closely, and when she detects the slight presence of something intangible on the ground, she crouches down onto her heels and stretches a hand out but pauses before touching.

"Yeah -- they're really delicate. Can I touch them?" she asks, genuinely interested. "They're butterflies, right?"

She's gotten good at sensing fine details, so she puts that together, but she's not totally confident.