The dentist will ask you a number of questions to begin with at the start of the appointment. This will give them information about the problem at hand and what may be causing it. Examples of the questions the dentist may ask you are discussed in diagnosing toothache
Root canals are necessary when the nerve has been irreversibly damaged or has died. The signs and symptoms above are those we generally associate with such situations. Some of the signs are a lot more definite than others. A swollen face is a pretty obvious sign of an abscess and infected tooth and the need for an extraction
or root canal treatment. So too is finding a large area of infection on an X-ray (see below) or a discharging sinus next to the root of a tooth.
The difficulty comes when the state of the nerve is more questionable. If the symptoms suggest irreversible pulpitis, whilst a sedative dressing can calm things down, the nerve in this state is likely to continue to die until root canal treatment becomes necessary. Sometimes, leaving it may make the root canal treatment more difficult in the long run, but at the same time you don't want to dive in there unnecessarily. The dentist may tell you the filling is deep and that it may need a root canal or extraction later. In these circumstances, it is being assumed that tooth has reversible pulpitis, but if the tooth was to become painful a few months or even years later, it would suggest that it was in fact irreversible and the nerve subsequently died.
Unfortunately the symptoms seen clinically (by the dentist) when pulpitis is present, vary quite a lot, particularly in multi-rooted molar teeth and pain is a very subjective sensation, (tending to affect males more than females)!
Whilst there is always an area of uncertainty, serious pain lasting a long time and keeping you awake at night suggests the nerve is irreversibly damaged. Pain from cold and sweetness, lasting only a few seconds suggests a reversible condition if it is dealt with promptly. If left however, this may progress and when more irreversible symptoms begin appearing, then you may well be looking at a root canal not just a filling- so it is best to get checked out quickly when you first notice anything.
This is not an exact science: reversibly and irreversibly damaged nerves are related to a spectrum of symptoms, which try to aid us in providing the most appropriate treatment. The tooth is a living tissue and will respond differently in different people and the diagnosis will almost always involve some element of uncertainty.
There are other less common reasons for needing a root canal and these are discussed in why you might need a root canal