There is a lady in Goldenrod City who will check your Pokemon's happiness. Her house is east of the Dept. Store and north of the Bike Shop (see outlined in green below).

She will make a comment about your Pokemon, which indicates its happiness:
- 250-255 → "It looks really happy! It must love you a lot."
- 200-249 → "I get the feeling that it really trusts you."
- 150-199 → "It's friendly toward you. It looks sort of happy."
- 100-149 → "It's quite cute."
- 50-99 → "You should treat it better. It's not used to you."
- 0-49 → "It doesn't seem to like you at all. It looks mean."
Your Pokemon needs 220 points to evolve next time it levels up.