A geo-fence is a predetermined radius around a project or specific location. A project's geo-fence can be changed in the project settings.
(Dashboard > Project > Settings > Info > Geofence)
Check if a user has used the check-in feature within the radius by looking at their timecard status.
Another way of setting the default radius for geo-fences is through the time module setting.
(Dashboard > Settings > Modules > Time > General Settings > Default Geofence Radius)