Is there a symbol on the pokemon? I don't think I could tell say, Gengar, by eye
On the first page of the pokemon's individual menu, there'll be a red star symbol next to the Galar mark, in the same row as the customizable markings, directly underneath the how many exp needed to next level info.
Fake Edit:
It should also appear as shiny during the raid so you will already see it when the fight starts
I think it only showed up as shiny in the post-raid selection screen, not during the fight. I was only half-paying attention but the shiny color would have been pretty obvious in this case.
Real edit:
If they do show up shiny during the raid, there's also the sparkle effect at the start of the battle as additional tip-off to go by for pokemon with very minimal differences.