As lemonade pointed out, history will be given that suggests this. For delirium, signs of systemic illness such as fever, electrolyte imbalance, and the like will be present. Urosepsis in the elderly is a notorious example. The context of substance withdrawal, or post-op period may be used. Odds are patient will be elderly, possibly on anticholinergics, and this will be in the hospital setting. The boards like to paint stereotypical pictures, so there'll be hints along the way. Impaired consciousness (unlike dementia or psychotic episode), autonomic dysfunction and other signs of outright physical illness, symptom exacerbation at night (sun-downing) will likely be present. It's unlikely you'll be given a question with no crutches at all, where you may as well flip a coin. 🙂