Every XHTML document is validated against a Document Type Definition. Before an XHTML file can be properly validated, a correct DTD must be added as the first or second line of the file.
Once you are ready to validate your XHTML document, you can use W3C Validator to validate your document. This tool is very handy and helps you to fix the problems with your document. This tool does not require any expertise to perform validation.
Following is the text box, you need to give complete URL of the page which you want to validate and then click Validate Page button.
This validator checks the markup validity of Web Documents with various formates specially in HTML, XHTML, SMIL, MathML, etc.
There are other tools to perform different other validations.