How do you calculate your pokemon's happiness in Pokemon HG/SS?

Question

There is no Poketch in HeartGold and SoulSilver, so how do you calculate it?

Answer 1

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.

Answer 2

If you go to a house in the Eastern Wing of Goldenrod City, you'll find a house where a girl will tell you how much your Pokemon like you. I'm not sure of exact phrases, but I think the highest-rated phrase she'll say is something along the lines of, "Wow, this Pokemon has a lot of trust in you!".