Where can I find a complete list of search criteria methods?

Where can I find a complete list of search criteria methods?

In the API documentation for CRM API method "SearchRecords"  there is only reference to two examples of a search that use the following :

&criteria=(((Last Name:Steve)AND(Company:Zillum))OR(Lead Status:Contacted))

it is obvious that the colon is used to represent equals.  But what other options are there?  Where is there a better definition of these?  Specifically I am in need to define a value as "Is Empty" since you can't use a blank value in the call or the use of null.