Dry Mouth or Xerostomia (the medical name), is defined as reduced or absent saliva. Dry Mouth is not a disease, but it may be a symptom of various medical conditions such as head and neck radiotherapy, or a side effect of a wide variety of medications. It can be associated with decreased salivary gland function.
Dry Mouth is a subjective condition usually worsening with age. There are many clear signs that a person can observe on themselves to decide if they have dry mouth. Clinicians (dentists or doctors) can also undertake "sialometry", a simple procedure that measures the flow rate of saliva. They can also take breath measurements with a gas chromograph for volatile breath compounds and perform an oral examination for a more definitive diagnosis.

SYMPTOM - Do you have difficulty eating biscuits and crackers. If dry foods are difficult to swallow then you are no producing enough of the lubricating aid "mucin". You may also notice that you are getting more frequent mouth sores when you eat.
SYMPTOM - Does the skin of your mouth look extra dry. Look at your tongue in the mirror, does it look fissured, dry or sore. Have the taste buds on the tongue surface atrophied (gotten smaller). Next look at your lips. Are they dry or cracked. Are you also getting cracking in the corners of the mouth.
SYMPTOM - Do your lips stick to your teeth when talking. Or if you are a female, does your lipstick seem to rub off onto your teeth more frequently. Do you get sore throats, dry nasal passages, or do you have difficulty speaking without having to sip water. Is your saliva frothy or thick, rather than the normal liquid look and feel. If you wear dentures do they cause you more issues such as sore spots, lack of retention.
SYMPTOM - Dry mouth tend to be worse between meals. Also at night, or first thing in the morning on waking. Do you find that your morning breath is worse on waking, or do you have bad breath (halitosis). Bad breath and morning breath are common signs of xerostomia as the lack of saliva promotes the production of volatile sulphur and organic compounds that have many different smells.
SYMPTOM - Do you have an intolerance to strong mints, flavours or foods. Do these things cause a burning mouth or sore tongue. Lack of saliva can promote and cause such issues. Have you been to the dentist lately and been diagnosed with new tooth decay when you have not had any change to your diet or circumstance. Have you been recently diagnosed with periodontal disease. Or do you seem to be getting more cases of oral thrush (oral candidiasis), or mucositis ( a painful inflammation of the gullet and throat).