Wham should you go to the doctor if you have a cold?

Depends.. .. On what your symptoms are. Typical URI symptoms are runny nose, cough, sneezing, head fullness/headache, maybe body aches... And this is a viral infection. You need to get plenty of sleep, stop smoking (if needed), drink fluids like water or dilute juices, and try to avoid infecting others. Call your doc for persistent fever, trouble breathing, chest pain, dizziness, or severe headache.
Med tx. Get medical care for temperature >104, difficulty breathing, worsening symptoms or if symptoms are not resolving within about 10 days. If symptoms worsen you may have a complication (i.e., pneumonia, bronchitis, sinusitis, or otitis media).