To determine which version of the .NET framework an assembly supports, you can use ILDASM.
Open up a visual studio command prompt, and type the following:
ildasm.exe C:Yourdll.dll /metadata[=MDHEADER] /text /noil
You'll get a large amount of indecipherable data, but right at the top, you'll see something to the effect of
// Metadata section: 0x424a5342, version: 1.1, extra: 0, version len: 12, version: v4.0.30319
where the highlighted piece gives you the supported version.
Alternatively, you can just open ildasm (just type ildasm at the command prompt), and open up the dll - you can see the metadata version right at the top by double-clicking "MANIFEST":