How can you tell? Do a search online for the manuals (Owners and service) and read them.
Try these:
http://s30387.gridserver.com/partsDiagrams/McCulloch%20Late%20models.pdf
http://lawnandgarden.manualsonline.com/manuals/mfg/mcculloch/mc3516.html?p=1
Anon
June 2016