How do you know one has max affinity with a character?

[AlwaysOnPoint]

^

[Forblaze]

There'll be a pink aura radiating out of their head. They'll also start to blush when you get close.

[Asterozoa]

loud chime sound, pink aura, pink blush.

Overview
The "beloved" is the character with the highest affinity for the Arisen - if more than one character has maximized affinity then the most recently spoken to of that group will be chosen as the beloved.

Once the quest Deny Salvation is accepted, the character who is deemed to have the closest relationship with the Arisen will be kidnapped by The Dragon and taken to The Tainted Mountain, and will disappear from the game until the completion of that quest. On completion the 'beloved' will move to live with the Arisen in Cassardis. On completion of the end-game the current beloved will appear in a final cutsene with the Arisen's former pawn.

Knowing exactly who your beloved is can be difficult. Even after they are taken by the Dragon, sometimes the only clue that this has actually happened is a short comment by the Dragonforged, if an NPC has clearly vanished from the world where they used to always be or if the Arisen Bond that was gifted to a character is dropped by Grigori at the Arisen's feet at the end of Deny Salvation. Eventually your beloved is revealed during your final encounter with The Dragon, at which point you are given the choice to sacrifice them or fight The Dragon to save the beloved's life. Once you kill The Dragon, the beloved will live in your house in Post-Game.

Beloved determination is not based on gender and it is entirely possible for your beloved to be the same gender as the Arisen.

Arisen's Bond
The Arisen's Bond is an item intended to be gifted to the Arisen's beloved; the item gives the receiver Maximum Affinity and so marks whoever you give it to as your beloved. If the Arisen's Bond was gifted, an extra cutscene will be shown after Deny Salvation showing that same ring drop from the sky - indicating that something has happened to your choice of beloved - despite the significance of this cutscene, the Arisen's Bond does not guarantee your choice of beloved will be chosen.

Beloved[dragonsdogma.wikia.com]