To check if a user is an administrative-level user, use the session_admin() function:

<?php

if (session_admin ()) {
    // administrator
} else {
    // not an administrator
}

?>

Similarly, you can call this function from a SimpleTemplate file via:

{if session_admin ()}
    administrator
{end if}
{if else}
    not an administrator
{end if}

And finally, you can call the same thing from an XT template via:

<xt:condition>
    <xt:if expr="php: session_admin ()">
        administrator
    </xt:if>
    <xt:else>
        not an administrator
    </xt:else>
</xt:condition>