private Func<TransactionScope> _getCurrentScopeDelegate;

bool IsInsideTransactionScope
{
    get
    {
        if (_getCurrentScopeDelegate == null)
        {
            _getCurrentScopeDelegate = CreateGetCurrentScopeDelegate();
        }

        TransactionScope ts = _getCurrentScopeDelegate();
        return ts != null;
    }
}

private Func<TransactionScope> CreateGetCurrentScopeDelegate()
{
    DynamicMethod getCurrentScopeDM = new DynamicMethod(
        "GetCurrentScope",
        typeof(TransactionScope),
        null,
        this.GetType(),
        true);

    Type t = typeof(Transaction).Assembly.GetType("System.Transactions.ContextData");
    MethodInfo getCurrentContextDataMI = t.GetProperty(
        "CurrentData", 
        BindingFlags.NonPublic | BindingFlags.Static)
        .GetGetMethod(true);

    FieldInfo currentScopeFI = t.GetField("CurrentScope", BindingFlags.NonPublic | BindingFlags.Instance);

    ILGenerator gen = getCurrentScopeDM.GetILGenerator();
    gen.Emit(OpCodes.Call, getCurrentContextDataMI);
    gen.Emit(OpCodes.Ldfld, currentScopeFI);
    gen.Emit(OpCodes.Ret);

    return (Func<TransactionScope>)getCurrentScopeDM.CreateDelegate(typeof(Func<TransactionScope>));
}

[Test]
public void IsInsideTransactionScopeTest()
{
    Assert.IsFalse(IsInsideTransactionScope);
    using (new TransactionScope())
    {
        Assert.IsTrue(IsInsideTransactionScope);
    }
    Assert.IsFalse(IsInsideTransactionScope);
}