Right now there is (see http://www.zabbix.com/documentation/2.0/manual/appendix/api/item/get)
"Possible values:
0 - a plain item;
1 - a discovered item."

but I'm quietly sure that there should be 4 values (from sources):

define('ZBX_FLAG_DISCOVERY_NORMAL', 0x0); // a normal item
define('ZBX_FLAG_DISCOVERY', 0x1); // a low level discovery rule
define('ZBX_FLAG_DISCOVERY_CHILD', 0x2); // an item prototype
define('ZBX_FLAG_DISCOVERY_CREATED', 0x4); // an item created via a discovery rule