Another option is to use PEiD ( I mentioned about it in some part II or III of this series. )
It's the *most* effective option.
It works not only for .NET apps, but for all kinds of EXEs, even encrypted, packed ones too.
Download PEiD and use it to examine the exe. If it's a .NET one, PEiD will show "Microsoft Visual C# / Basic .NET". Not only that, it gives loadz of useful info about the exe.