The SOCKS protocol traffic is usually logged with as ACCELERATED under the s-action field. For example:
#Fields: date time time-taken c-ip sc-status s-action sc-bytes cs-bytes cs-method cs-uri-scheme cs-host cs-uri-port cs-uri-path cs-uri-query cs-username cs-auth-group s-hierarchy s-supplier-name rs(Content-Type) cs(Referer) cs(User-Agent) sc-filter-result cs-categories x-virus-id s-ip
2009-06-16 09:00:48 3 10.105.1.65 0 ACCELERATED 12 13 CONNECT tcp 216.52.23.9 80 / - - - NONE - - - - PROXIED "Computers/Internet" - 10.10.10.10
The following fields can also be added into the Access Log for the SOCKS protocol
x-cs-socks-ip
x-cs-socks-port
x-cs-socks-method
x-cs-socks-version