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.