Indeed, the compression process GZIP / DEFLATE is transparent to the user , so that browsers display the same way the websites that are compressed and not (and thankfully so, because if need the decompressed for us without us having to do anything. currently most of the pages are sent compressed and a delay would have to decompress hand )
So if you want to see if a page is compressed or not , the right way is to examine the HTTP headers , to look at the response from the web server, which has a flag that indicates whether and how compressed format. It is as simple as reading these headers, but of course as browsers will not show by default, because we have to have some tool to view them.
In the HttpLiveHeaders Firefox plugin is very interesting and useful.
We can also access this link , where they have a form where you enter an address and say whether or not the response was compressed with GZIP / Deflate and get various data related attractions such as the size of the compressed file uncompressed .