How can I see which Delphi version a .dcu was compiled with?

The DCU file is a proprietary format developed at Borland (now Embarcadero) as the intermediate data between compiled code and the source code. It is the main reason why the Delphi compiler can generate an executable so fast. Informations about this format are intentionally scarce. Borland did not provide documentation about it, Embarcadero seems to follow the same path. There is an utility called "DeDe" that can disassemble compiled files created by old Delphi versions and bring several informations about these, including the dcu file version. It does not work for binaries produced by Delphi compilers newer than version 5, but the source code is included, maybe it could be improved.

While looking for the answer, I ran into an awesome online tool by Alexei Hmelnov.

It lets you upload a .dcu file, and it'll give you extremely detailed information about it.

http://geos.icc.ru:8080/scripts/WWWBinV.dll

It shows the compiler that was used, the compile time, used units, etc.

From unofficial sources, look at 4th byte of the .dcu

$0F = Delphi 7
$11 = Delphi 2005
$12 = Delphi 2006 or 2007
$14 = Delphi 2009
$15 = Delphi 2010
$16 = Delphi XE
$17 = Delphi XE2
$18 = Delphi XE3
$19 = Delphi XE4
$1A = Delphi XE5
$1B = Delphi XE6
$1C = Delphi XE7
$1D = Delphi XE8
$1E = Delphi 10 Seattle
$1F = Delphi 10.1 Berlin
$20 = Delphi 10.2
$21 = Delphi 10.3
$22 = Delphi 10.4

There was no change in .dcu format going from Delphi 2006 to Delphi 2007. Therefore they use the same.

Edit Jul 2, 2016 Added XE8, 10 and 10.1 to the list.

On request, also the target platform, which is found in the second byte of the .dcu. Values are of course valid only for versions that have these targets.

$03 = Win32
$23 = Win64
$04 = Osx32
$14 = iOS emulator
$76 = iOS device
$77 = Android

Let me know if you think there's an error.