*Topic moved to 'Help for New Tamagotchi Pet Owners'*
As far as I know, there isn't a way to check before you buy it. The way we discover that a certain food is a character's favorite is when you scroll to it in the food menu (after buying it), your character (who is standing next to the food shown, if I remember correctly) will have an excited look on his/her face and raises his/her arms up (or something similar to this; you'll know). Also, after your character finishes eating it, the word "GREAT!" will show up.