Its not difficult to be required to file Form 568 if your business has any connection to the State of California.
California has pretty good resources on its website, including the links above.
Who Must File (taken from 568 instructions, page 6)
An LLC may be classified for tax purposes as a partnership, a corporation, or a disregarded entity. The LLC should file the appropriate California return. Form 568 must be filed by every LLC that is not taxable as a corporation if any of the following apply:
• The LLC is doing business in California.
• The LLC is organized in California.
• The LLC is organized in another state or foreign country, but registered with the California SOS.
• The LLC has income from California sources (Nonregistered foreign LLCs, see Exceptions to Filing Form 568, below).
An LLC is not required to file a tax return and is not subject to the annual taxes and LLC fees if both the following are true: • The LLC’s taxable year is 15 days or less. • The LLC did not conduct business in the state during the 15 day period.