When you get a good heart, the NPC you talk with makes a kissing animation.
When you get a broken heart, the NPC you talk with starts laughing.
If you are mounted and talk with an NPC, he will make the animation (kiss or laugh) but it will say "you are mounted" as an error. So now, you know if he will like you or not. If he likes you, unmount and get it. If he laughs at you, go to the next one