In order to be protected from legal liability (and, in my personal opinion, to practice with integrity) you must work within your scope of practice in the manner that you have been trained. In the hypothetical situation you presented, you are making a diagnosis and doing an invasive intervention that could potentially cause serious harm to the person if you are wrong, or do the intervention improperly.
"Potential complications of NT include cardiac tamponade, life-threatening bleeding due to pulmonary artery or intercostal vessel injury, nontherapeutic (i.e., ineffective) NT insertion, and nerve injury/neuralgia at the insertion site" (Complications of needle thoracostomy: A comprehensive clinical review).
Or, if the patient doesn't have a true tension pneumo and while inserting the needle you cause a laceration to the lung, the patient can develop an air embolus.
Additionally, it can be difficult to properly diagnose the condition without ultrasonography. One study I ready on the subject said that approximately 30% of patients who were treated for tension pneumo in the prehospital setting actually didn't have it. Also keep in mind that the signs/symptoms of tension pneumo are not always going to be textbook, and that given the mechanism of injury there can be significant comorbidities that could make the accurate diagnosis in the prehospital setting very difficult without the proper resources and training.
As far as your reference to the trachea:
"Late and unreliable signs: Increasing tension may include tracheal deviation (or tracheal "tugging") and jugular vein distension (JVD). Simple visual inspection is unreliable in detecting tracheal deviation. Bates' guide to physical examination recommends placing your fingers on either side of the trachea to determine if the distance between the trachea and the sternocleidomastoid muscle is equal bilaterally. Prehospital Trauma Life Support (PHTLS) indicates the trachea is bound to the cervical spine and fascia, and deviation would be detected lower, at the sternal notch" (Tension Pneumothorax | EMS Reference).
"In this case, providers would simply feel the deficiency of the trachea if it has shifted to one side. These authors also point out that JVD might not be present either. If tension pneumothorax results in decreased cardiac output with hypotension, it is not likely to cause JVD. Note that although tracheal deviation is rare, it is primarily a sign of tension pneumothorax. In contrast, JVD can be present in multiple conditions, such as cardiac tamponade, so it is not necessarily an obvious tension pneumothorax finding" (Tension Pneumothorax | EMS Reference)
The point of both of those quotes from the same article is to show you how detailed and specific diagnosing it can be.
Without extensive training I don't think it would be appropriate for a nurse do that on a patient. Additionally, I don't think its ethical to have the patient decide if they want you to do it or not by asking them to nod their head. You are the medical professional, you know the limits of your training and the patient may not or may not be in a position to make an informed and rational decision, especially while scared and in pain.
What would I do in this situation? I like the idea in the end of the article
"EMS providers should maintain good situational awareness and not fixate on one problem. If mechanism existed to create a tension pneumothorax, then other traumatic injuries may also be present. Thorough assessment and monitoring of the patient's condition is also essential. Look for other causes, injuries and co-morbidities" (Tension Pneumothorax | EMS Reference). I would do everything within my training and ability, and do everything in my power to get them to help. Anything outside of that seems to be reckless and you'd be putting them, as well as your license, at risk.
But, hey, thats just my opinion.