Yes! You can see it's shiny form even for a Pokémon you haven't caught. The same applies to the forms too. A Pokédex will record Location data, cry, appearance (Variations in ♂️ and♀️, other forms, shinies) for a Pokémon you have seen. You only need to catch it for getting details on Pokédex entries, Footprint, species. Since you need the data of Form/shiny changes, making a Pokémon appear in battle is only enough.
(This applies to all games including Sw/Sh)
Source : My experience