This really depends on the individual. There will naturally be some suspicion when first confronted with a bowl of new food, however providing it is introduced gently and sensitively virtually all cats and dogs will grow to love their new diet. There is a grain of truth in the feeling that they sort of 'sense' or 'know' that their new food is doing them good. 

One technique worth using if there is an initial resistance, is to lightly cook the food in the microwave on its lowest setting. I am sure many cat and dog owners give their pets the occasional treat such as a piece of meat left over from the Sunday roast. The smell of the lightly cooked food will get their interest up. This can be done over the first few days, reducing the amount of cooking so eventually it is just warmed slightly but not cooked at all. Within a short time your pet will be relishing his or her new food - we can assure you!