As per google places API documentation, no authentication is required for using the API. Is that how even check-ins to a place works? number of check-ins to a place are application dependent or aggregation of all check-ins across all registered apps.
Why checks-ins API is not provided via Google Places Javascript API
Using the Google Places API requires you to authenticate each request using an API Key.
Any Places API Check-In Requests sent using the Places API will only affect subsequent Places API Search Request for the API Key used to issue the Check-Ins.
As the Places Library of the Google Maps JavaScript API v3 does not require authentication to use, Check-Ins are not part of the library.