In templates:
{% if user.is_superuser %}
<p>Hello, admin.</p>
{% else %}
<p>Hello, ordinary visitor.</p>
{% endif %}
In views:
if request.user.is_superuser:
# Hello, admin.
else:
# Hello, ordinary visitor.
Depending on your needs, is_staff might be a better fit than is_superuser. You can read about the difference here.