If you are looking at the engine it should be on the left side panel also called your timing belt or t-belt cover as some call it. Look on the side of that and it will have the numbers there.
The 2006 Volvo S60 has an inline 5 engine.
The 2001 Volvo S60 has an inline 5 engine.
The 2002 Volvo S60 has an inline 5 engine.
The 2004 Volvo S60 has an inline 5 engine.
The 2008 Volvo S60 has an inline 5 engine.
The 2003 Volvo S60 has an inline 5 engine.
The 2015 Volvo S60 has an inline 4 engine.
The 2007 Volvo S60 has an inline 5 engine.
The 2009 Volvo S60 has an inline 5 engine.
The 2011 Volvo S60 has an inline 6 engine.
The 2014 Volvo S60 has an inline 5 engine.
The 2012 Volvo S60 has an inline 5 engine.