How to find local disk space by `df` command?

$ df -lh would do the work.

Where [from man page],

-l, --local - limit listing to local file systems

-h, --human-readable - print sizes in human readable format (e.g., 1K 234M 2G)

df -l should restrict the output to local file systems.

Alternatively, if you know that the device name of the local disk is, for example, /dev/sda, then you can use grep to filter out just those disks: df | grep sda.