xml sitemap module : XML declaration allowed only at the start of the document [closed]

Question

I installed the xml sitemap module, when I go to my sitemap.xml page, it give me an error:

This page contains the following errors: error on line 1 at column 6: XML declaration allowed only at the start of the document Below is a rendering of the page up to the first error.

How can I find spaces that shouldn't be there? How to find spaces before <?php tag, but omit files where they are legitimate, like theme files?