Determine framework (CLR) version of assembly

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.

One clarification...

The problem with all the mentioned methods is that they will return version 4.0 if assembly was compiled against .NET framework 4.0, 4.5 or 4.5.1.

The way to figure out this version programmatically at runtime is using the System.Runtime.Versioning.TargetFrameworkAttribute for the given assembly, for example

using System;
using System.Linq;
using System.Reflection;
using System.Runtime.Versioning;

...    

object[] list = Assembly.GetExecutingAssembly().GetCustomAttributes(true);
var attribute = list.OfType<TargetFrameworkAttribute>().First();

Console.WriteLine(attribute.FrameworkName);
Console.WriteLine(attribute.FrameworkDisplayName);

Will return

a.FrameworkName ".NETFramework,Version=v4.0"    string
a.FrameworkDisplayName  ".NET Framework 4"      string

a.FrameworkName ".NETFramework,Version=v4.5"    string
a.FrameworkDisplayName  ".NET Framework 4.5"    string

a.FrameworkName ".NETFramework,Version=v4.5.1"  string
a.FrameworkDisplayName  ".NET Framework 4.5.1"  string

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.

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.