Look up a parts place online and look at parts that tend to vary from model to model, such as carbies, bodywork, electrical components, speedos.. It's pretty easy to identify which model you have. Also, some parts lists have a feature where you can look up a part number, and then click a link somewhere that shows you what other models that parts fits. This can be handy to see which other models do have that part your bike has, before going o to explore those other models and comparing their parts with what you have. If the first spares site you look up doesn't have that cross reference feature, it's worth looking for a spares site that does have it.

I believe that the top ends for those Honda 400/450's were interchangeable & people were doing the swap for more power. Not sure how to tell the difference in cyl. heads but using the 400 head would provide more compression. You can go to an online Honda parts place to determine what jetting came stock and compare to what is in yours. Another thing to check is the VIN (serial)# to see what model the frame is - maybe the fenders were switched? There are lots of aftermarket parts out there these days so finding out what you have gets tricky.