From USA WEEKEND Magazine:

Here are the signs of a fungal nail infection — just remember the word YUCK:
Y Yellow discoloration of the nail bed
U Under the nail bed, a buildup of skin
C Cracking of the nail
K Keratinization, or thickening of the nail

You are always best advised to go to the doctor. However, the treatment for nail fungal infections is medication that is extremely harmful to your liver and you will need to have your liver checked periodically if you choose to take the medicine. It is also a long term medication that you often must take for nearly a year. Some home remedies that I have used for fungal infections are Lotrimin cream, Vicks VapoRub and Tea Tree Oil. Use each twice a day. And if the infection gets worse, go to a doctor immediately.