You can either:
Have Poketch App #6, which tells you your pokemon's happiness when you press and hold it by the number of hearts that appear above it. If it has two large hearts, it means it loves you the most, whereas no hearts mean its happiness is 0.
You can also go to Dr. Footstep's house on route 213. If your pokemon's happiness is at max, he will give it a ribbon.
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level up your GOLBAT with high happiness.
It evovles with max happiness
It evolves through happiness
Level it up when it its happiness is full
Any level, just get it to max happiness.