I also found that if your trying to validate a url path against a document or alias in the given site, you can use:
CMS.DocumentEngine.PageInfoProvider.GetPageInfoForUrl(url, null, null, true, true, SiteContext.CurrentSiteName);
I tried using
CMS.PortalEngine.PageInfoProvider.GetPageInfoForUrl(...); but didn't seem to work. I may have been inheriting the wrong classes though or this changed since in version 9. Just another options!