How to tell AutoCAD File Format

If a user opens up a dwg file created in a newer version of AutoCAD, chances are the following error will appear:

Drawing File is not Vaild (error)

You can view a given DWG file in Notepad and look at the string at the beginning of your file, to match the version it was created in from the list below.

MC0.0 - DWG from Rel. 1.1
AC1.2 - DWG from Rel. 1.2
AC1.4 - DWG from Rel. 1.4
AC1.50 - DWG from Rel. 2.0
AC2.10 - DWG from Rel. 2.10
AC1002 - DWG from Rel. 2.5
AC1003 - DWG from Rel. 2.6
AC1004 - DWG from Rel.9
AC1006 - DWG from Rel.10
AC1009 - DWG from Rel.11/12 (or LT R1/R2)
AC1012 - DWG from Rel.13 (or LT95)
AC1014 - DWG from Rel.14, 14.01 (or LT97/LT98)
AC1015 - DWG from AutoCAD 2000/2000i/2002 (or LT, Map or relative Desktop version)
AC1018 - DWG from AutoCAD 2004/2005/2006 (or other product in the "2004", "2005" or "2006" family)
AC1021 - DWG from AutoCAD 2007 (or other product of the "2007" family)

There are also other, internal file versions: AC1001=pre-2.5, AC1005=pre-R10, AC1007=pre-R11, AC1008=pre-R11, AC1010=pre-R13a, AC1011=pre-R13b, AC1013=pre-R14, AC1500=pre-2000, AC402a=pre-2004a, AC402b=pre-2004b