if type xcode-select >&- && xpath=$( xcode-select --print-path ) &&
       test -d ${xpath} && test -x ${xpath} ; then
       #... is correctly installed
    else
       #... isn't correctly installed
    fi

Strangely, the `--print-path` isn't documented in old Xcode versions but is working as `-print-path`. On the other hand, the `-p` option doesn't
have this compatibility.