How do you tell if the food is a gourmet food or not?

I don't believe such a feature exists. What do you need gourmet food for? Do you mean like filet mignon and lobster and stuff?

edit: If you're talking about the achievements "Gourmand" any food that has an option to feed (marked as food in the database) counts. "Unique" just means you can't feed it the same item multiple times to increase the score.

There is an achievement to feed X numbers of unique r99 foods I think.

Here is the list

Fancy Food and Foodie are the two Achievements for feeding your pet r99 items. If you go for the other Achievements that are named "gourmand" you will get these two Achievements along the way. If you want to find out which foods are r99 then both the fansite Subetalodge and Subeta itself have Item databeses that will let you sort for them. Subetalodge is more databasey in that it allows you to also sort by cheapest to most expensive at the same time.

Gourmet as far as the Achievements that say Gourmet is concerned, that's any food at all, like sez. Pets on Subeta can always be made to eat if they're hungry.

Thank you to all of you for the information :)