You probably don’t think about your tongue too much. Sure, you use it when you eat an ice cream cone or blow a raspberry, but other than tasting food, your tongue doesn’t seem like its very useful.
But the truth is, your tongue can tell you some amazing things about your health and give you a warning sign to let you know when you should see a doctor. Here are some health conditions that your tongue can help diagnose.

White Film, Coating or Spots: This could be a sign of a yeast infection inside your mouth. The patches will remind you of cottage cheese and are more common in those with compromised or lowered immune systems.
Leukoplakia: This is an overgrowth of cells in the mouth that could become cancerous. Smokeless tobacco users are particularly at risk.
Red Tongue: A red tongue (as opposed to the normal pink color) can indicate a vitamin deficiency such as B12.
Red & Bumpy: This could be an indication that you have an infection such as scarlet fever. If the tongue symptoms are accompanied by an actual fever, you should see your doctor right away. If the condition is present on a child younger than five, it could be an illness called Kawasaki Disease and you should see a doctor right away.
Blackened Appearance and Hairy: This could be a bacterial infection within the papillae of your tongue. Generally, this is caused by poor dental hygiene and is not usually serious.
Painful Bumps: There can be a few different causes for this. Burning or biting your tongue can cause it to become bumpy or painful and smoking can contribute to tongue pain as well. Some painful bumps may be canker sores or ulcers of the mouth. All of these conditions are not usually serious and will generally heal on their own. If you have a lump on your tongue that doesn’t go away within two weeks, you should definitely see a doctor, even if you aren’t having pain. Oral cancer can present without pain in the early stages and you want to catch it as quickly as possible.
Webbed or Striped Appearance: This could indicate a condition called lichen planus which may or may not be painful but should be monitored by a doctor because the areas affected have a higher risk of oral cancer.
Ridges on the Tongue: If you have ridges or scalloped edges you might just be biting down and creating the edge on your tongue. Those who were born with a fissured tongue may also notice ridges. However, this could also be a sign of an autoimmune disease so you should see your doctor.
Smooth Surface or Pale Appearance: This might also be a sign of a B12 deficiency or a sign that you need more iron.
Swollen Tongue: If your tongue feels as if it has swollen or looks larger than normal, it could be a sign of hypothyroidism, which is when your body doesn’t produce enough of the thyroid hormone. Lower energy will be another symptom.
Sources:
http://www.prevention.com/health/health-concerns/what-white-tongue-means-and-other-weird-symptoms
http://health.clevelandclinic.org/2015/04/what-your-tongue-can-tell-you-about-your-health/
http://www.womenshealthmag.com/health/tongue-health-warnings