XML and DTD

Hi,
I don't know xml very well and my question is stupid!!!
I have created an file xml and another file dtd for that
xml.
When I call my xml file from my browser (IE)
I can see it well formatted. But I don't know how I must do
to see if it is "VALID".
I have inserted into my xml file this row:

Because if I have understood well
"stanalone" identify with "yes" if I want that the check
is only "well fomatted" and with "no" if the check
is "valid document".

But the result is equal. And If I add some error into
my xml file I haven't any segnalation!

How I can do to know if my xml file have the rule of
his dtd????

Please, help me!
Thanks in advance and sorry for my little english!

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