The knock sensor hole is there on e30 as well, only difference will be if they left the knock sensor in, if it is the hole I think it is. If I get a chance I can check tomorrow, I have an e36 block in the garage. We have a plug there I think.
Only way to tell is buy serial number.
If it is a new enough e36, it will likely have different cam sprockets. That would require pulling the valve cover. Keep in mind, my e30 has e36 sprockets because of them being changed.
Why does this even really matter? These motors last a long time with proper care. Pull the lower pan, check the bolts and call it good.