To answer your question...
The only way we can truly pronounce someone on scene is if there are obvious signs of morbidity/injury not compatible with life. These include decapitation, decomposition, and rigor mortis. Special circumstances apply during mass casualty incidents when, based on START triage criteria, a patient is pulseless and apneic after 2 rescue breaths and resources are needed to manage other critical/living patients.
In special circumstances we can contact OLMC for the doc's call on the situation. These may include extremely large patients or physical barriers (extrication, tech rescue, wilderness) that place extenuating factors on patient transport.
We do not pronounce pregnant patients or hypothermia related cardiac arrest. We are encouraged not to call pediatric codes in the field unless they are obviously DOA.
For prehospital termination of resuscitation (non-traumatic), patient generally must be 18+, pulseless and apneic, asystole on the monitor, full ACLS followed, and no ROSC for at least 2 minutes. Any sign of neuro activity, we work it til the ER.
Trauma codes are simple in my book. When I arrive on scene, if they have a pulse or are breathing, scoop and run and treat en route. If they have already gone pulseless/apneic prior to my arrival, call the ME. If they code en route, it becomes a skill session. We automatically do intubation, bilateral NDT, dual IVs, NG, etc.
If in doubt, transport.