In a hypothetical world where using TLS was abnormal, you could monitor the content of whatever the bots are doing for suspicious activity. And if they chose to use TLS anyway, the mere presence of of TLS could be considered suspicious.

Back in the real world, you can also passively fingerprint TLS handshakes to characterise the client device. Most of these proxy networks masquerade as "normal" clients, but if the type and variety of device fingerprints for an IP suddenly changes, that's a signal too.