The tool used by an operating system (OS) to administrate addition, removal and upgrade of software applications. Examples of package management systems on different OSes are: Linux: RPM; Solaris: pkgadd; HP-UX: SD-UX; Debian: apt-get; Win32: Windows Installer
If your system uses a package management system, you can query that system's database to see if the attr package and its associated library, libattr, are installed.