If you do not have access to AutoCAD you can check the version number of a DWG file by opening it in NotePad.  The first 6 characters denote the version as follows (red indicates file versions that are not supported by Stardraw 2007 applications):

AC1024 - 2011
AC1024 - 2010
AC1021 - 2007/2008/2009
AC1018 - 2004/2005/2006
AC1015 - 2000/2000i/2002
AC1014 - r14
AC1012 - r13
AC1009 - r11/12
AC1006 - r10
AC1004 - r9
AC1003 - v2.60
AC1002 - v2.50
AC1001 - v2.22
AC2.22 - v2.22
AC2.21 - v2.21
AC2.10 - v2.10
AC1.50 - v2.05
AC1.40 - v1.40
AC1.2 - v1.2
MC0.0 - v1.0

All later versions of AutoCAD (i.e. 2007 onwards) can save back to earlier versions of the DWG file format, so if you cannot open a DWG because of version incompatibility simply ask the originator to save it back as DWG 2004/2005.

Kind regards,
Rob Robinson
Stardraw.com