How do I determine the original fan number so that I can see if this is compatible? Where is that info usually located?

Thank you for your question on the FG600 Grille Upgrade. There should be a label, stamp or etching on the wall of the housing (metal box in the ceiling) or mounting plate that would provide the model information to you. This label is generally visible by removing the Grille completely. In some models, it may be necessary to remove the motor as well. Please reach out to our Customer Service Team at 800-637-1453 if we can be of assistance.