ALWAYS CALL FOR UTILITY LOCATES!
I can't stress this enough. Before you dig, call a locate company. Check in the beginning of your phone book for a number to call. They will usually locate for free about 48 hours from when you call.
If you have a property located, and you hit a unmarked wire/pipe/etc you cannot be held liable for damages.
Remember, one damaged utility can potentially take all profit from the job, and very easily leave you in the red.
Once the locate is done, the general code is this:
Red = Power
Orange = Cable or Telephone
Blue = Water
Green = Sewer
Yellow = Gas
Always hand dig across the locate lines, giving 2' on each side before you use any machines. A locater is very accurate, but you can never trust the operator. In my State, if you hit a line within 2' of the locate mark, you still are liable.