You can check/use the system variable OPENFORMATVERSION (values below). Using DWGNAME you get the used file extension DXF or DWG.
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Values of the opened file format
4  FLX – FelixCAD 4
9  FLX – FelixCAD 5
11 FLX – PowerCAD 6
14 FLX – PowerCAD 7
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9  DWG/DXF - AutoCAD R 2.5
10 DWG/DXF - AutoCAD R 2.6
11 DWG/DXF - AutoCAD R 9.0
13 DWG/DXF - AutoCAD R 10
16 DWG/DXF - AutoCAD R 11 / 12 (R12)
19 DWG/DXF - AutoCAD R 13 (R13)
21 DWG/DXF - AutoCAD R 14 (R14)
23 DWG/DXF - AutoCAD R 2000 (R15)
25 DWG/DXF - AutoCAD R 2004 (R18)
27 DWG/DXF - AutoCAD R 2007 (R21)
29 DWG/DXF - AutoCAD R 2010 (R24)
31 DWG/DXF - AutoCAD R 2013 (R27)
33 DWG/DXF - AutoCAD R 2018  (R32)