Go to the Hanged Man and speak to Isabela. Hawke will give the necklace to Isabela, who explains that the necklace is a Rivaini fertility necklace that is meant to increase fertility and bring about love.
If you ask her if she knows much about talismans then in the next conversation choice "I don't see anything wrong." is no longer an option.
"I knew what it was." or "I don't see anything wrong." (+10) with Isabela
"Oops. I see it now." (+10) with Isabela
Note: Even if you have romanced Isabela fully and she claims she has fallen for you, she still acts hesitant about love, saying that she'd rather stick with lust.
If Hawke is male and you flirt with Isabela then you go upstairs and have sex - if Hawke is female then she simply thanks you for the gift.