You're going to have to get a strong light source, shine it from the rear, and use a magnifying glass to peer at the trouble spot until your eyeballs fall out. My guess is that it isn't fungus, but this is only a guess based on your one low rez photo here.
Seriously, if you look very closely w/ a good light source and a good magnifying glass you should, eventually, be able to tell what it is. If it IS fungus, set it in the sun for a day or three w/ the lens pointing directly at the sun (you'll have to keep shifting it now and then). Watch out for setting things on fire under it, so make sure you do this on a brick or in a driveway. That should stop the fungus in its tracks. It has on every lens I've tried it on. It will still be there but it won't get any worse. Have you tried shooting it w/ a good hood? I'll bet it has no effect on image quality, but it WILL affect what you can sell it for later. Time for a partial refund from your seller.
