Ykls is a utility for listing connected Yubikeys, along with some relevant information, such as enabled modes, firmware version, serial number, identities present in the various key slots, ...

For example: $ ykls Reader: Yubico YubiKey FIDO+CCID 00 00 Version: 4.3.7 Serial: 6447364 Slot Authentication (9a): nicoo@debian.org