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.
President and CEO - Napmoor and Doolittle.
2004 Mercury 270 Dinghy.
2016 Grand Design Reflection 29RS 5th Wheel
2016 GMC Sierra 2500HD SLT 6.8L Diesel
1995 Carver 325
1999 Four Winns 268
1999 Four Winns 225
1996 Rinker 180