How to determine whether Apache is using as Forward Proxy or Reverse Proxy?

Apache can be used as Forward Proxy or Reverse Proxy, how to determine whether Apache is using as Forward Proxy or Reverse Proxy? I think it is configed in httpd.conf file, but I don't know which configuration field decide this.

Solution

ProxyPass and ProxyPassReverse are the directives that need to be configured to set up Apache as a reverse proxy.

In simple terms 'ProxyPass' performs unidirectional address space conversion from external to internal as in :

ProxyPass       /app1/  http://internal1.example.com/
ProxyPass       /app2/  http://internal2.example.com/

so http://www.example.com/app1/some-path maps to http://internal1.example.com/some-path as required.

while ProxyPassReverse performs the inverse translation from the app/web -server response to the externally address space as in :

ProxyPassReverse /app1/ http://internal1.example.com/
ProxyPassReverse /app2/ http://internal2.example.com/

this enables self-references/references to other internal servers to not pass through as-is but to be converted to external address spaces, in case of re-directs for example:

    HTTP/1.1 302 Found  
    Location: http://internal.example.com/foo/         
 //ProxyPass lets this through to user browser as-is!

with Reverse Proxy this gets returned to the user's browser as

  HTTP/1.1 302 Found
  Location: http://www.example.com/foo/

using a ProxyPassReverse directive.