Like raw turkey, you can tell when cooked turkey has gone bad by the smell. Cooked turkey should smell like the seasonings you used in preparing it. But when this smell fades and is replaced by a sour odor, the turkey is unfit for consumption.
Cooked turkey may also become slimy when bad. As hinted before, the slime on turkey (cooked or raw) is produced by bacteria. Therefore, one should never taste suspicious turkey to verify its freshness. Doing so opens one to possible enteritis.