With Blender V3.1+ you can use the Domain Size node in that way :

The output of the Math node can be used as a boolean since it will return 0 if there a no evaluated instances in the Object Info node. You don't even need the Math node since the instance count will evaluate to True for values > 0.

With Blender 3.0 I don't know of a very straightforward way. I'll assume the object contains some geometry with at least 2 vertices. You can use the Attribute Statistic node :

The output of pretty much any socket of the node will evaluated to True if there are at least 2 vertices in the mesh.

With Blender V3.1+ you can use the Domain Size node in that way :

The output of the Math node can be used as a boolean since it will return 0 if there a no evaluated instances in the Object Info node. You don't even need the Math node since the instance count will evaluate to True for values > 0.

With Blender 3.0 I don't know of a very straightforward way. I'll assume the object contains some geometry with at least 2 vertices. You can use the Attribute Statistic node :

The output of pretty much any socket of the node will evaluate to True if there are at least 2 vertices in the mesh.