Calling doctor about crying
Many illnesses, infection and injuries in young children and babies often have crying as a major symptom, especially because babies cannot tell you if something hurts or when they don't feel well. For this reason, it is better to be safe than sorry if your baby is experiencing prolonged episodes of inconsolable crying, and visit your GP sooner rather than later.
Refusing to eat or sleep, diahrroea, vomiting, skin rashes, fevers (over 38 degrees C) and slow weight gain are not symptoms of colic and should be assessed by a doctor. Colic can't be diagnosed based simply on a baby's history of crying - there may be an underlying cause, and this should be investigated by a doctor to rule out physical causes such as reflux or intestinal obstruction.
Your GP will do a physical exam of your baby, but if your baby is otherwise healthy, then blood tests, x-rays, and other diagnostic tests aren't usually needed to diagnose colic. However, if your doctor suspects that something else is the cause of your baby's prolonged crying episodes, such as gastroesophogeal reflux, a hernia, a urinary tract infection, or a neurological condition, they may do more extensive testing.
One way to help your doctor to either diagnose or rule out colic is to keep a diary of your baby's crying over a week, noting what times they started and stopped crying, how long each episode lasted, what measures you tried to comfort them, and what (if anything) worked. When you're an exhausted new parent with a newborn, this may seem like a huge pain in the neck - but it's well worth the trouble and can help your GP to more effectively recommend treatment.