Topic on Talk:The big list of third-party DRM on Steam
In the case of SolidShield a game will either activate silently during installation (as with the original Crysis 2) or display an activation prompt. Games that don't do either aren't using the activation feature.
DRM types with dedicated removal tools (e.g. SecuROM) are easier since the tool can be used to show what DRM data is present; if a removal tool doesn't detect anything after running a game that means it isn't using the activation feature since even silent activation will install DRM data that the tool can then detect.