How to find what MPM model Apache is using in Linux (worker, prefork or event)

httpd -V

then look for the Server MPM line.

Example:

# httpd -V
Server version: Apache/2.2.15 (Unix)
Server built:   Aug  2 2013 08:02:15
Server's Module Magic Number: 20051115:25
Server loaded:  APR 1.3.9, APR-Util 1.3.9
Compiled using: APR 1.3.9, APR-Util 1.3.9
Architecture:   64-bit
Server MPM:     Prefork
  threaded:     no
    forked:     yes (variable process count)
Server compiled with....
 -D APACHE_MPM_DIR="server/mpm/prefork"