If you want to check your pages without DTDs then try [validator.w3.org...] as that has selector boxes that allow you to set the !DOCTYPE and the Character Encoding for the page without needing it actually on the page.
However, long term, you should aim to add these elements to your code on every page.
You might also want to consider what else you put in the header. For most people, code on each page begins something like:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<title> Your Title Here </title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta http-equiv="Content-Language" content="EN-GB">
<meta name="Keywords" content=" your, keyword, list, here ">
<meta name="Description" content=" Your Description Here. ">
<meta name="MSSmartTagsPreventParsing" content="TRUE">
Code within the page:
I use: <a href="somepage.html" title="some text here"></a> for links.
I use <img src="somefile.png" alt="some text"> for images.
Headings are done with <hx></hx> tags, properly used from <h1></h1> downwards.
[edited by: g1smd at 2:11 am (utc) on June 18, 2003]