Evasion protection is a built-in feature. Some evasion protection is always active in packet processing but mostly it is an inspection feature, since aim of evasions anyway is to hide attacks, and inspection is the feature to block these.
High security inspection policy gives better antievasion capability compared to medium security. On the downside, high security policy is more likely to give false positives with production traffic.
There is a free evasion testing tool available in http://evader.mcafee.com that can be used to test any inspecting device's antievasion capability.