how to detect a callout of an annotation is showing on mapview?

I have a checkin related app. When user clicks a venue on map, callout will popup and show its name and address. At the same time, if the venue is within 1000m from current location, a check in button will be displayed.

To a venue that can checkin, all I want is,

when callout is popup, check in button shows. when callout is disappear, that button disappears too.

Now show button works well, but I don't know how to detect callout visible status. Is there any callback method that callout show/hide?

Thanks in advance!