how to find layer type of a layer

How do I find a layer type of an ILayer? For example, if I loop through all ILayers from an IMap, how do I determine whether each ILayer is an IGroupLayer ?

I already know I can use IMap.get_Layers(<IUID>, true) to obtain all the layers of a given type according to the passed IUID.

But I prefer to first obtain all layers, then determine what type of layer it is.