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.