How many materials are available in BOM ?
You can use the Std report CS11 and CS12 to know how many BOM Componenets of a FERT BOM have BOMs. Goto the tcode CS12 and in the selection screen enter the material / plant / BOM Application and execute. System will display the BOM structure the Materials which have ..2 under it indicate that that particular compoennt does have the BOM under it.
Option 2 :
If you have very little FERT material then goto tcode ; CS03 and display the BOM for the FERT material and check of the Field Assembly Indicator . If the check box is populated ... then it indicated that it does have a BOM under it.
Hope this will help you.