You can specify an event role on an event for the one who is going to check in volunteers.  For example, have a role 'Lead Volunteer' that you then specify as the check-in admin.  When a volunteer signs up for that role, they will get an email a few days prior to the event.  That email will have a link to check in volunteers.  Volunteers who signed up for the role to check in volunteers, will have the ability to view/manage volunteers for that event.  After the event passes (1 day after) they will no longer be able to view/manage the volunteers.