Chest pains are cardiac until proven otherwise. Headaches are subarchnoid hemorrages until proven otherwise. Specialists are always awake at 3 am. Remember to call them for consultations. Patients who return to the ER department after visiting earlier in the day usually have something seriously wrong.
Patients are usually quite smart about their own diseases.
When in doubt, admit the patient. Senior citizens are always sicker than they appear or state. Teenagers are always pregnant and usually will deny it until asked three times. You will only regret not having done it later that night when you go home. The worry is not worth it.