The little icon in the lower right corner of a Smart Object Layer looks the same whether it is a single layer or more than one layer inside the Smart Object Layer. Is there a way to tell at a glance if more than one layer is wrapped inside the smart object layer?
Photoshop renders pixels for the smart object layer's object the object if the ojbect has one layer or thousands of layers. These pixels are not changeable with Photoshop tools. They can only be operated on by Photoshop filters and the associated smart object layers transform. Only Place illustrator files like .AI and .SVG files are vector smart object layers. Most smart object layer are resized via interpolation for the associated smart object layer's transform. There is no way the tell what the layers object is without double clicking on the smart object layers content icon in the layers palette to open the layer's object so you can work on the actual object.
Is there a way to tell at a glance if more than one layer is wrapped inside the smart object layer?
As JJ hinted, No.