How to determine the priority of css files in info file for a theme?
The CSS files are processed in order, so if your .info file looks like this:

stylesheets[all][] = css/foo.css
stylesheets[all][] = css/bar.css


Then the styles from foo.css will be included before those from bar.css. The same applies whether each CSS file is included separately, or whether they're aggregated. 

If you're still unsure which rule will be applied at that point, you need to read up on CSS specificity, as Drupal doesn't affect it.