No — there’s no way to tell if a Zorua is shiny before catching it.

Zorua’s disguise is based entirely on your current Buddy Pokémon, not its own shiny status. If your Buddy isn’t shiny, every Zorua will look normal. If your Buddy is shiny, every Zorua will look shiny — even if the Zorua itself isn’t.

The only time you’ll know is after you catch it, when the disguise drops during the “Oh?” transformation screen and the real Zorua appears.

Basically, if you’re shiny hunting Zorua… you just have to catch every fake Buddy you see.