You can iterate through the Layers and Linetypes collections to find all the layers and linetypes in a drawing.
The following code iterates through the Layers collection to gather the names of all the layers in the drawing. The names are then displayed in a message box.
Dim layerNames As String
Dim entry As AcadLayer
layerNames = ""
For Each entry In ThisDrawing.Layers
layerNames = layerNames + entry.Name + vbCrLf
MsgBox "The layers in this drawing are: " + _
vbCrLf + layerNames