How to find out which version of the .NET Framework an executable needs to run?

Using Notepad, three decades old, 200kb in size, preinstalled tool:

• open application with notepad appname.exe,
• search for word framework,
• repeat last search with F3 until .NET Framework,version=vX.Y shows up
• if nothing found (versions below 3.0) search for v2. ... still 100 times easier then installing gigabytes of dot net analyzer tools and garbage studios.

update: Thought and Marcus suggested in comments that search term could also be netstandard or netframework

You can now use ILSpy to examine the target framework of an assembly. After loading the assembly, click on the root of the assembly node, and you can find the information under the TargetFramework declaration:

[assembly: TargetFramework(".NETFramework,Version=v4.5", FrameworkDisplayName = ".NET Framework 4.5")]

A more simplified approach would be to use dotPeek and see what shows up in the tree.

See the properties panel:

I think the closest you can reliably get is to determine what version of the CLR is required. You can do this by using ILDASM and looking at the "MANIFEST" node or Reflector and looking at the dissasembly view of the "Application.exe" node as IL. In both cases there is a comment that indicates the CLR version. In ILDASM, the comment is "// Metadata version" and in Reflector the comment is "Target Runtime Version".

Here are examples for a .NET WinForms application named WindowsFormsApplication1.exe:

ILDASM:

// Metadata version: v2.0.50727
.assembly extern mscorlib
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 2:0:0:0
}
.assembly extern System
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 2:0:0:0
}

Reflector:

.module WindowsFormsApplication1.exe
.subsystem 0x0002
// MVID: {CA3D2090-16C5-4899-953E-4736D6BC0FA8}
// Target Runtime Version: v2.0.50727

You can also look at the list of referenced assemblies and look for the reference with the highest version number.

Again, using ILDASM looking at the "MANIFEST" node data:

.assembly extern System.Drawing
{
  .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A )                         // .?_....:
  .ver 2:0:0:0
}
.assembly extern System.Core
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 3:5:0:0
}

And using Reflector, looking at the dissambly (still as IL) for each reference listed:

.assembly extern System.Core
{
    .ver 3:5:0:0
    .publickeytoken = (B7 7A 5C 56 19 34 E0 89)
}

By finding the reference with the highest version metadata you can determine what version of the Framework that reference came from, which would indicate that you need the same version of the Framework installed for the application to run. That being said, I wouldn't treat this as 100% reliable, but I don't think it will change any time soon.