I've seen similar questions for java and VB.NET, but nothing general for .NET, or specific to C# yet.
In the past, I've decorated methods with the Obsolete attribute (passing true so the compiler will issue an error, as described in MSDN).
I'd be interested in seeing the suggestions of others (beyond tools like FxCop or ReSharper).
Edit: The reason for asking about tools other than FxCop or ReSharper is that I want to make sure I'm not missing out on other tools that would be helpful.