Look for a oil filler cap. If it exists, its likely a 4 stroke. However, many 2 stroke engines also have oil injection for their larger engines - whether or not it includes 15hp or not, I am not sure. For many of the smaller 2 stroke engines, you still have to mix oil yourself (I have to with my 2004 Mercury 5hp dinghy motor)
I am going to guess though that it is a 2 stroke, as I don't think Johnson makes any small 4 stroke engines.
But I don't know for sure, so best thing is to look up the model number on-line or consult a dealer.