The best method is to use a Sparkling and Encounter Power Psychic sandwich on the beach West of Levincia.
Specifically on the side closest to the wall and the watchtower. Although the shade can possibly throw you off, I found this spot to be the best, as the Slowpoke spawn easier on this side compared to the other, and compared to other locations, the Slowpoke spawn closer together, allowing you to compare them if you aren't sure.
Using Level 3 Encounter Power Psychic gets you just Slowpoke on this beach (the other side closer to Levincia has trouble spawning Slowpoke because of the hillier terrain), the beach next to the Porto Marinada Lighthouse, and on the land parts surrounding Casseroya Lake. I found the beach near Levincia and Casseroya to be the most efficient, even though Slowpoke sometimes spawn in groups with Slowbro and are a bit more spread out at Casseroya.
For the best odds, you're going to want the Shiny Charm, Sparkling Power Psychic 3, and 60+ Slowpoke KOed in an outbreak for shiny odds of 1/512. However, I didn't find getting an outbreak to be necessary, so just Sparkling and Encounter Power should get the job done if you don't have the charm. The sandwich recipe I used was 1 serving of Pickles, 1 serving of Cucumbers, and 3 servings of Onions, with 2 different herba mystica that weren't a combination of Sweet and Sour, but if you have a different recipe, feel free to use that instead. Save before making the sandwich so that if you don't get any shinies, you can reset the game and get your herba back.
If you're really that worried about missing the shiny, the Masuda Method with the Shiny Charm also get you odds of 1/512, although at that point, it would likely be faster to just hunt Slowpoke in ZA.
As for telling the Slowpoke is shiny, there isn't really much you can do. Compared to the more purple shiny in Legends ZA, shiny Slowpoke in SV is more pale, so it's just one of those shinies you're going to have to know what it looks like and keep an eye out. I recommend avoiding the shade if you aren't confident, hunt at night if you need to (the rock wall near at the Levincia spot creates a shadow that disappears at night), and hunt in an area where the Slowpoke are closer together so you can easily compare them. Additionally, if you target a shiny with your autobattler, your Pokemon won't KO the shiny, so if you aren't sure, just try to autobattle the Slowpoke. Just be careful not to KO too many and end the outbreak if you are hunting in one.
That's all the tips I have for you. Good luck on your hunt, and hope this helped!
