There's an old saying, "The patient who diagnoses himself has a fool for a client.
" While I don't think that's entirely true, it can be a very dangerous practice.
You may be OK checking to see if a trip to the doctor is a good idea, anything else is a bad idea.
Here's why.

• Similar Symptoms: Dehydration and liver disease can both cause dark urine, but the severity of the conditions are vastly different.
  Checking with your doctor is a good idea, as liver disease can kill you.
  
• Generic Descriptions: A website can't list every single symptom that can ever occur.
  The symptoms will most likely be very generally described and limited to those most likely to occur.
  An example would be heart attacks.
  There are some very unusual symptoms that can indicate a heart attack, but you're not likely to find all of them on a website.
  
• Mom (or dad) has it, therefore I must have it: Heredity can be involved with many illnesses, but that does not mean that you will have every single illness ever experienced in your family.
  Tell your doctor your family history and let the doc figure out if you have risk factors for medical conditions.
  You'll need to pay proper attention to symptoms, but constantly worrying over it is not good for your health.
  
• Missed Signs: This is something that happens frequently, and most likely to everyone.
  Some symptoms are very subtle and easily missed.
  Unless you've had sufficient medical training, it's hard to know that the color of your nail bed or the slight ache in the back can be important.
  The doctor can, and will ask questions based on other symptoms that you had noticed.
  
• Lack of test results: There are some medical conditions that require medical tests to diagnose.
  Cancer, diabetes and heart disease all need to be confirmed with tests, both to be sure what is happening and to decide how to treat it.
  

For these reasons, it really is best to see your doctor in order to have a proper diagnosis.
Using family history and web sites to see if the symptoms mean something may be useful, but the final say should be in the docs hands.