<?php echo S2MEMBER_VERSION; ?>
This may output something like: 110620
(or whatever the current version number is)
Use PHP's version_compare() function to test this.

Note: s2Member versions now follow their release date.
Ex: s2Member v110620 was released June 20th 2011.
 (YYMMDD format)

---- s2member Shortcode Equivalent ----

[s2Get constant="S2MEMBER_VERSION" /]