One of the best signs of healthy conflict is sincere laughter. If people don't take themselves too seriously and are willing to recognize how foolish and silly they can be at times, they are far less vulnerable to taking offense or holding grudges.
Another sign is that people are willing to admit their own shortsightedness and acknowledge when others' ideas may be better than their own. This is why the apostle Paul urged the Philippians to "do nothing out of selfish ambition or vain conceit, but in humility to consider others better than yourselves" (Phil. 2:3).
A third sign is that people come back to discuss an issue, not so they can re-argue their own point, but to ask thoughtful questions to clarify what someone else said.
Finally, if people can have different views on one issue, but still respect and engage each other on other issues, their conflict is probably still in the healthy and productive zone.1