If enabled, it is the orange button on their radio. Every radio that's used in public service has one, although the system must be properly configured to support it. Some RSMs (I believe all of the new Motorola APX accessories) will have one on there as well. Additionally, there are various other forms of activation that can be enabled on a specific subscriber, such as horizontal sensor/man down or remote emergency button.
Basically whether or not it will work, and what happens when you press it, is entirely up to the programming in the subscriber unit and the setup of the system. All systems, even analog conventional, are capable of some form of emergency call/alert if properly configured, but often they are not.