[K12OSN] squid question

[mrojava4 at eastgranby.k12.ct.us]

Does a web server such as apache know whether the traffic it is receiving
is via a proxy server (such as squid) or directly from a source PC?

The reason that I am asking is that we recently switched ISP's from Cox
Cable to the CEN (Connecticut Education Network) and we have run into some
problems going through our squid proxy server versus just using nat/masq. 
Certain pages that cause userid/password popups seem to hang going through
our proxy server, but work fine when the proxy server is bypassed.

I'm wondering whether, if there is a difference between proxy output
versus direct PC output, the new ISP may be doing some (improper)
filtering.

Mark Orenstein
East Granby, CT School System