Can I get Facebook fan page-id in page tab app without authorizing user using javascript? And also if I authorized user and after that I get signed_reques
No, you can not.
The page info is send only in the signed_request that gets POSTed to your app so with JavaScript, you have no access to POST data. But you can easily access with server side language.
You might found a lots of PHP code for it..for C# you can use below code..
string signedR = Request["signed_request"];
I hope this might be helpful to you...@Mirant