Tongue Diagnosis is a part of the Chinese Medicine Diagnosis. The tongue tells the Chinese Medical Practitioner many things in the theory of Oriental Medicine and is considered to be an excellent source of information. You have to know and understand Chinese Medicine Theory to really understand what the tongue is telling you, but there are some Western Medical things we can sometimes extract from the tongue. Here is a small rundown of some things you might be able to correlate out of the tongue.

The tongue is a muscular organ associated with the function of deglutition,taste and speech.It acts as an easily accessible organ for the assessment of  health of an individual and shows the state of hydration of the body. It is said that tongue is the mirror of the gastrointestinal system and any abnormal functioning of the stomach and intestines will be reflected on the tongue.
Some characteristic changes occur in the tongue in some particular diseases.That is why the examination of the tongue can be quite useful and will give some clues for diagnosis. Some doctors examine the tongue and things to consider would be: size, shape, color, moisture, coating, papillae and any movements

Here is some appearances that would flag diseased conditions:

Involuntary movement of the tongue:

-In one sided paralysis of the body(hemiplegia)tongue moves towards the paralyzed side when extended out.

-Tremors of the tongue is seen in diseases like thyrotoxicosis,delirium tremens and parkinsonisum. Tremors can also give away nervousness in a patient.

-Bulbar palsy will show wasting and paralysis of the tongue with fibrillation.Eventually shrivel up and lies functionless in the floor of the mouth.

Tongue Moisture:

Moisture will first and foremost show the hydration of the body. Water volume depletion leads to peripheral circulatory failure characterised by weakness,thirst,restlessness,anorexia,nausea,vomiting ,dry and parched tongue.

Tongue Dryness:

-Diarrhea
-Prolonged illness
-Hypovolumic shock
-Heat exhaustion
-Food Stagnation
-Starvation
-Prlonged fasting.

Tongue Color Changes:

-Cyanosis

Cyanosis is a bluish coloration of the top layer due to decrease in the amount of oxygen in the blood. This can be seen in heart failure,respiratory failure and in anemia. Also note that the lips are typically blueish in Cyanosis.

-Jaundice

Jaundice will normally show up as yellowish almost all over the body due to increase of bilirubin in the blood. Jaundice is seen in hepatitis,bile duct obstruction,increased destruction of RBCs and ect. Jaundice should be easily noticed in other areas first like skin, sclera, etc.

-Keto acidosis

This is the acidosis with accumulation of ketone bodies seen mainly in diabetes mellitus. Here the tongue become brown with a typical ketone smell from the mouth.

-Riboflavin deficiency

Deficiency of this vitamin (vitamin B2) produces megenta colour of the tongue with soreness and fissures of lips.

-Niacin deficiency

Deficiency of niacin (vitamin B3)and some other B complex vitamins results in bright scarlet or beefy red tongue.

Anemia:-

Anemia is a lack of oxygen bonding to blood cells. The tongue can show up as pale usually in this condition.
Coating of the Tongue

-Bad breath

The main cause for bad breath, halitosis, is a formation of a pasty coating(bio film) on the tongue which lodges thousands of anaerobic bacteria resulting in the production of nasty breath. Those who complain about bad breath may have thick coating on the root of the tongue.

Candida

Candida is a fungal infection which affects the mucus surfaces of the body.On the tongue there will be sloughing white lesions.

-Secondary syphilis

Syphilis is a sexually transmitted diseased caused by trepenoma pallidum infection.In secondary stage of this disease we can see mucous patches which are painless,smooth white glystening opalescent plaques which can not be scraped off easily.

-A hairy tongue

This condition is due to elongation of filiform papillae seen in poor oral hygeine ,general debility and indigestion.

-Geographic tongue

Here irregular red and white patches appear on the tongue.These lesions looks like a geographic map.The excact cause is not known, although many are born with this condition, so they should be questioned if their tongue has always been like that.

-Nutritional deficiency

In nutrional deficiency there is glossitis(inflammation of tongue) leading to papillary hypertrophy followed by atrophy.

-Vitamin A deficiency:-

This causes furrowed tongue.

-Scarlet fever;-

In this streptococcal infection there is bright red papillae standing out of a thick white fur ,later the white coat disappear leaving enlarged papillae on the bright red surface and is called strawberry tongue.

Tongue Ulcerations:

-Apthous ulcer

These are round painful ulcers appear in stressed individuals frequently. May be associated with food allergy.Usual sites are tongue,lips,oral mucosa and ect.

-Herpes simplex

It is an acute vesicular eruptions produced by herpes simplex virus.When these vesicles rupture it forms ulcers.

-Ulcer in cancer

Cancerous ulcers are having everted edges with hard base.Bleeding is also seen.Cancer of the tongue is common in tobacco chewers.

-Syphilitic ulcers

Syphilitic fissures are longitudinal in direction.In primary syphilis extra genital chancre is seen on the tongue.In secondary syphilis multiple shallow ulcers are seen on the under surface and sides of the tongue.In tertiary syphilis gumma may be seen on the midline of the dorsum of the tongue.

Well, thats all the information I know of and can find on the net about the tongue and such. The tongue is an extremely accurate way to diagnose in Chinese Medicine, but is it that useful in Western Medicine? Well, who knows but maybe it can answer some questions you may have had about the look of your tongue :)