I am using nginx as a reverse proxy and trying to read a custom header from the response of an upstream server (Apache) without success. The Apache response is the following:

HTTP/1.0 200 OK
Date: Fri, 14 Sep 2012 20:18:29 GMT 
Server: Apache/2.2.17 (Ubuntu)
X-Powered-By: PHP/5.3.5-1ubuntu7.10
Connection: close
Content-Type: application/json; charset=UTF-8
My-custom-header: 1

I want to read the value from My-custom-header and use it in a if clause:

location / {
    // ...
    // get My-custom-header value here
    // ...
}

Is this possible? Thanks in advance.

It's not only possible, it's easy:

in nginx the response header values are available through a variable (one per header). See http://wiki.nginx.org/HttpCoreModule#.24sent_http_HEADER for the details on those variables.

In your examle the variable would be $sent_http_My_custom_header.