Hi Tuy,
Thank you for contacting Syncfusion support.
We do not support providing list of markers available in the viewable area in the Maps control. The coordinates of the viewable area's corners in the Maps control are also not exposed in the Maps control. However, we have considered this as a feature and added it to our features request list. We do not have immediate plans to implement this feature. It will be available in any of our upcoming releases. Please find the feedback link below to keep track of the feature.
Regards,
Sabari Anand.