The number one would be shipping product (or anything really) out in front of a customer.
If you can't work together to get something done fast, that is usually a huge indicator that somethings wrong. That usually means your values or mission aren't aligned.
I've been a leader/manager of small teams at companies of various size over the past decade.
The early symptoms of conflict I've seen can seem innocent at first. Things like people not eating lunch together anymore, forming cliques, not participating in group activities, showing up late for work or not showing up at all.
The early symptoms of a team breaking down are subtle - not unlike cracks in a dam. It usually starts with erosion in Trust which shows up in communication break downs, seeking out who to blame, looking out for #1 or being too busy to help each other out.
What I look for is when someone comes to me as the Leader looking to share information about another team mate. If the information gets their team mate "out of trouble" they're working as a team; if the information gets their team mate "in to trouble" then the dam is starting to break.
In my experience working with elite athletes and seeing dynamics played about between the Athlete-Coach relationship, or overall sports teams and entrepreneurs I've consulted, Great teamwork should be fostered in an environment of open-mindedness and a mandate embraced that idea-conflict is healthy, but personal conflict will not be tolerated.
Too often though weak leaders, or uninformed leaders, believe their teams must all become friends with one another and they attempt to force this camaraderie rather than let it develop naturally.
More specifically, early symptoms of unhealthy personal conflict include members needling one another behind the other's backs, negative body language (one of the most relevant signs, after all it's difficult to hide in a workplace), and a third symptom whereby one begins to repetitively shifting blame toward the other for even the smallest of tasks not being fulfilled - or perceiving the task as being completed at a level not to their satisfaction.
Feel free to ring me for an initial consultation...the beginnings of a fractured team dynamic will certainly be detrimental to the overall success of the teams goals. This is a topic I've dealt with many times in my career, and I'd be happy to provide you with actionable steps to resolve the issue and provide immediate steps to rally the team so you can achieve your mandate.
Probably the number one symptom is "triangulation." That is, I have a problem with Person A, but I go complain about it to Person B. Every management team should be aware of this dynamic and all agree on nipping it in the bud (which means Person B has to STOP the triangulator, and help them deal with the conflict directly).
Another sign is silence. If team conversations are usually lively but suddenly go quiet, there may be conflict. The number one response to conflict is avoidance. So have your radar tuned for the silence and learn to probe some when that happens.