There are a variety of ways to check in/out volunteers for their service. One of the ways is by an administrator. To allow this, first you need to allow Check in volunteers on the event. If this is enabled, when viewing the event as an administrator you will see a View Check-Ins link user the event roles. If the event has chosen to allow Administrator check-ins, there is an option on the event to specify 'Who will check them in?'. There you can search for a specific user, and we will send them an email with a link to check in the volunteers the day prior to the event.
By clicking on the check-ins, you will be taken to the administrator check in page for this event. There you can check in/out volunteers as they arrive. This screen will also show any volunteers who have not signed the waiver (if your instance requires waivers). You will only be able to check in/out volunteers on the day of the event.