How to tell if a layer is in a group layer

Is there a way to tell if a layer is in a group layer? I was writing out the datasource for layers in a map, which worked fine, but can't tell if a layer is in a group or not. Can test whether a layer is a group layer or a feature class. Some layers are in group layers, some not. Some in group layers in group layers. This isn't a big deal, but would be nice to format the output in a directory tree style. Thanks.