We will not be able to see which calendar getting used by system during SLA execution, because business days calculation process related data will not get stored inside case (or) anywhere, it is a kind of fly-over process. Business day's calculation will be done based upon the below mentioned points.
In functions if we hardcoded the business calendar name, then system will use the provided calendar for calculating business days.
If business calendar name was not provided then system will make use of Default calendar for calculating business days.
If you want to make use of particular calendar getting used during the SLA execution, then in SLA action process first step set the .pyCalendar property value to the calendar name which you want system to use for calculating business days. Then instead of Default system will use the value provided in .pyCalendar property for calculating business days.