From Bulbapedia, the community-driven Pokémon encyclopedia.
The marking symbols (●, ▲, ■, ♥, ★, and ♦) on the Summary page of a Pokémon is a great way to mark which IVs are the Pokémon's best, preferably if they are Perfect (31).
HP: ● ATK: ▲ DEF: ■ Sp.ATK: ♥ Sp.DEF: ★ Speed: ♦
These may also be used to indicate Imperfect IVs (0) as well, although this is less common place. This is typically only reserved for use when a Pokémon exhibits more than one Imperfect IV.
For indicating a single Imperfect IV, an EV reducing berry can be held to indicate the stray IV. The Pomeg, Kelpsy, Qualot, Hondew, Grepa and Tamato berries are all useful tools in breeding and raising a Pokémon and can be readily multiplied via planting.
HP: ATK: DEF: Sp.ATK: Sp.DEF: Speed: