Determine framework (CLR) version of assembly

ildasm.exe will show it if you double-click on "MANIFEST" and look for "Metadata version". By default, it's the version that the image was compiled against.

class Program {
  static void Main(string[] args) { 
      System.Console.WriteLine(
             System.Reflection.Assembly.LoadFrom(args[0]).ImageRuntimeVersion);
  }
}

Compile and run the above application under the latest .NET Framework (as an older CLR may be unable to load assemblies requiring a newer CLR) and run it passing the path to the assembly you want to check as the command line argument.

Here's a PowerShell equivalent of the .NET code suggested in another answer. Using PowerShell means that you can skip a few steps like creating and compiling an assembly.

At a PowerShell prompt, run the following:

[System.Reflection.Assembly]::LoadFrom("C:\...\MyAssembly.dll").ImageRuntimeVersion

By default, PowerShell uses the .NET v2 runtime, so you'll get an exception for assemblies targetting v4. Stack Overflow question How can I run PowerShell with the .NET 4 runtime? details methods for changing that, if required.

Here is a powershell one liner that will display the Target framework version for assemblies targeting v4 and up.

 Resolve-Path($args) | Select @{N='Assembly'; E={$_ }}, @{N='TargetFramework'; E={(([Reflection.Assembly]::ReflectionOnlyLoadFrom($_).GetCustomAttributesData() | Where-Object { $_.AttributeType -like "System.Runtime.Versioning.TargetFrameworkAttribute" })).NamedArguments.TypedValue}} | Format-Table

use:

C:\test\> show-targetfw.ps1 *.dll

Assembly             TargetFramework
--------             --------
C:\test\a.dll        ".NET Framework 4.6.1"
C:\test\b.dll        ".NET Framework 4.5.2"

From command line

DUMPBIN your dll/exe /CLRHEADER

I use ILSpy as a replacement for Reflector. If you open the assembly in ILSpy, you can see, for example:

[assembly: TargetFramework(".NETFramework,Version=v4.6.2", FrameworkDisplayName = ".NET Framework 4.6.2")]