In Pokémon Let’s Go Pikachu and Let’s Go Eevee, your partner Pokémon will look a little different depending on whether it’s a male or female. Regular Eevee don’t look any different, regardless of whether they are male or female. But if you get a female Eevee as your partner, you’ll find that its tail seems to be marked by a heart-like pattern. This is only the case for partner Eevee.
Source: 【公式】プロジェクトイーブイ @project_eevee