Determine .NET Framework version for dll

Load it into Reflector and see what it references?

for example:

In PowerShell you can use the following to get the target runtime:

$path = "C:\Some.dll"
[Reflection.Assembly]::ReflectionOnlyLoadFrom($path).ImageRuntimeVersion

I adapted this to PowerShell from Ben Griswold's answer.

If you want to know the target framework version specified in Visual Studio, use:

$path = "C:\Some.dll"
[Reflection.Assembly]::ReflectionOnlyLoadFrom($path).CustomAttributes |
Where-Object {$_.AttributeType.Name -eq "TargetFrameworkAttribute" } | 
Select-Object -ExpandProperty ConstructorArguments | 
Select-Object -ExpandProperty value

You should get something like

.NETFramework,Version=v4.5.2

You can use ILDASM...

ildasm.exe C:\foo.dll /metadata[=MDHEADER] /text /noil

and check for the 'Metadata section' in the output. It would be something like this:

Metadata section: 0x424a5342, version: 1.1, extra: 0, version len: 12, version: v4.0.30319

The 'version' tag will tell you the .NET Framework version. In the above example it is 4.0.30319

dotPeek is a great (free) tool to show this information.

If you are having a few issues getting hold of Reflector then this is a good alternative.