In the case of mayo, it's lecithin, found in eggs.

partially hydrogenated in the ingredients list.

The current evidence suggests that, compared with Western nations, the intake of trans fats from partially hydrogenated oils may be much higher in developing countries, typically because partially hydrogenated oils represent the cheapest (and often subsidized) choice of fat for cooking.

Thus, little justification can be made for the use of partially hydrogenated oils, compared with other natural oils or fats.

Considerable evidence exists for harmful effects of trans fat intake. Furthermore, trans fats from partially hydrogenated oils have no intrinsic health value.

In summary, considerable evidence exists for harm, and little evidence for intrinsic health value, of consumption of trans fats from partially hydrogenated vegetable oils.

While small amounts of trans fats are naturally present in meats and dairy products from cows, sheep, and other ruminants, the great majority of trans fats in our diet are industrially produced, contained in foods made with partially hydrogenated vegetable oils.

Given that coronary heart disease is the leading cause of death in nearly all countries, including developing nations, intensive efforts must be undertaken to greatly reduce or eliminate use of partially hydrogenated oils in both the developed and the developing world.

But there are no hard and fast rules about how small a serving can be, and, that means, you'll have to rely on seeing those key words, partially hydrogenated, because that's how trans fats are made, by partially hydrogenating unsaturated fats.