How can you tell if a sylesti is themed when you fight them? It's in the fest area.

In the fest zone, if you encounter a themed pet, the log will say "You have encountered a Rare Enemy!" and the words "rare enemy" will be in purple, instead of red like usual.

Once you're in the fight, the pet will also display its theme name. For example, instead of a Wild Lupora, you might encounter a Summer Rose Lupora.

In addition to what Somneli stated, Themed pets always appear alone, and are always Blue star rarity or higher (Blue, purple, yellow, red) (that is, they always have at least two trait points, though they might be Carried not Vis)

Here are some images from one of our guides so you know what an encounter looks like: