How to tell if it's a datagrid

[Bob Sneidar]

Given an object reference, how can I tell the object is a datagrid? You might say, get the dgProp ["control type"], but every object contained within a datagrid group will ALSO return "data grid". In fact, EVERY dgProp arguement for every item in a datagrid group will do the same. 

The reason I ask is that I want to get the minimum left/top and maximum right/bottom of every VISIBLE object on a card that is NOT a group but notwithstanding INCLUDING Datagrids, so that I can set the rect of the card containing them in a consistent way. The thing is, groups have margins (I'm not going to set the margins of all my groups to 0) and these are not visible, so could skew the positioning of what IS visible. Datagrids do NOT have margins that I can tell, even though they are groups. For example, if I had a group on the left edge but not on the right edge, the objects on the card would look off center. 

Bob S