public class A
{
    public int AProp { get; set; }
}

public class B : A
{
    public int BProp { get; set; }
}

public void TestAction(A a)
{
    if (!(a is B))
    {
        throw new Exception("Should be B!");
    }
}

var setting = GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings;
setting.TypeNameHandling = TypeNameHandling.All;

{$type: 'MyNamespace.B, MyAssembly', AProp: 1, BProp: 2}

var json = @"{$type: 'MyNamespace.B, MyAssembly', AProp: 1, BProp: 2}";

var objA = JsonConvert.DeserializeObject(json, typeof(A), new JsonSerializerSettings
{
    TypeNameHandling = TypeNameHandling.None
});

var objB = JsonConvert.DeserializeObject(json, typeof(A), new JsonSerializerSettings
{
    TypeNameHandling = TypeNameHandling.All
});