You can use Reflector, ILDASM or ILSpy to get the assembly version.
You usually can find ILDASM in C:\Program Files (x86)\Microsoft SDKs\Windows\v8.1A\bin\NETFX 4.5.1 Tools\ildasm.exe (where v8.1A is the version of the Windows SDK installed).
ILDASM:
Reflector:
