If you start a project with .NET Framework in 2026, you're doing it wrong, plain and simple.

And the .csproj files do tell you which .NET they are.

<TargetFrameworkVersion>v4.</TargetFrameworkVersion> or <TargetFramework>net4</TargetFramework> is the old framework. Also, if the file is an unreadable mess listing all .cs files, it's generally .NET Framework.

<TargetFramework>netstandard2.0</TargetFramework> is .NET Standard 2.0, which means this library can be consumed from either Framework or modern .NET.

And finally, <TargetFramework>netX.0</TargetFramework> (X >= 5) is the modern .NET.