When you are eating a food item from an addon, It does not play the eating animation in the first person and so you can't tell if you are eating it unless you are in 3rd person where it does show the eating animation for your player model, Same goes for an add-on potion, in 3rd person you can see the drinking animation but not in the first person.

It also does not play the drinking or eating sound effect but it does play the eaten sound effect after you finish eating the addon food item.