The easiest way of getting there is to have no mutable state. And here is a simple test to ensure that. Seeing how CouchDB code works and how erlag handle things is quite educating in this regard.

[TestFixture]
public class EnssureTypesSafeForMultiThreadingTestFixture
{
    [Test]
    public void TypeIsSafeForMultiThreading()
    {
        var visitedTypes = new List<Type>
        {   // immutable types, partial list
            typeof(int),
            typeof(long),
            typeof(string),
            typeof(DateTime)
        };
        foreach (var type in GetRootTypesToCheck())
        {
            CheckType(type, visitedTypes);
        }
    }

    private static void CheckType(Type type, ICollection<Type> types)
    {
        if(types.Contains(type))
            return;
        types.Add(type);
        var fields = type.GetFields(BindingFlags.Instance|BindingFlags.NonPublic|BindingFlags.Public);
        foreach (var info in fields)
        {
            var isReadOnlyField = (info.Attributes & FieldAttributes.InitOnly)==FieldAttributes.InitOnly;
            if(isReadOnlyField==false)
                throw new InvalidAsynchronousStateException("Dude, " + type + "." + info.Name +
                                                            " is not marked as read only. You are NOT safe for multi threading, enjoy the deadlock, bye!");
            CheckType(info.FieldType, types);
        }
    }

    private static IEnumerable<Type> GetRootTypesToCheck()
    {
        // return types that I am interested in verifying
    }
}