To add to what Will was saying, she'll say any of these messages:
"It looks really happy! It must love you a lot." 250-255
"I get the feeling that it really trusts you." 200-249
"It's friendly toward you. It looks sort of happy."150-199
"It's quite cute." 100-149
"You should treat it better. It's not used to you." 50-99
"It doesn't seem to like you at all. It looks mean." 0-49
Your pokemon has to have a happiness level of at least 220 in order to evolve, which goes for any pokemon that evolves by means of happiness. So once she says,
"I get the feeling that it really trusts you," you know you're close.
Since you're playing HG/SS, make sure you get the soothe bell which helps raise happiness faster. And pokmeon that hatch from eggs start off with a happiness of 120.