In bash, there's also:

    command -V

Which knows about aliases and functions as well:

    $ command -V ip
    ip is /sbin/ip
    $ command -V ll
    ll is aliased to `ls -AlF --color=auto'
    $ command -V command_not_found_handle
    command_not_found_handle is a function
    command_not_found_handle ()
    {
        if [ -x /usr/lib/command-not-found ]; then
            /usr/lib/command-not-found -- "$1";
            return $?;
        else
            if [ -x /usr/share/command-not-found/command-not-found ]; then
                /usr/share/command-not-found/command-not-found -- "$1";
                return $?;
            else
                printf "%s: command not found\n" "$1" 1>&2;
                return 127;
            fi;
        fi
    }