This should be a safe way:

PageData pd;            
PageReference pr = new PageReference(PageId);                                    
if(!PageReference.IsNullOrEmpty(pr))                 
     pd = DataFactory.Instance.GetPage(pr);

if (pd != null)          
 {                
       // Your page exists and you have access to it          
 }

Hi,
Thank you for your respond,
I think that new PageReference create a new PageReference even if the page dosent exist.
And that mean PageReference.IsNullOrEmpty is not going to be NullOrEmpty becase the Reference exist but is not a valid page.

                if(PageReference.IsNullOrEmpty(pr))
                    return;

                try
                {

                    // I GOT A PAGE NOTFOUND exception here.
                    PageData pd = DataFactory.Instance.GetPage(pr);
                    pageList.Add(pd);                              
                }
                catch
                {
                }

There is one way to do it without the try catch, but I am not sure how supported it is.

you can use:

if(PageCoreData.Load(PageRef) != null)

{

//The pageRef is ok and exists.

}

PageCoreData.Load returns null if you have a page ref that leads to a page that does not exist instead of throwing an exception. I have not investigated it much though.. Like if it is slow /cached, ..., ... so might be some issues in some cases im sure.