Apple has plenty of documentation on this:
http://developer.apple.com/document...og_Guide/paste_intro/chapter_1_section_1.html
OSStatus err = noErr;
PasteboardRef inPasteboard;
PasteboardCreate(kPasteboardClipboard, &inPasteboard);
PasteboardSyncFlags syncFlags;
ItemCount itemCount;
syncFlags = PasteboardSynchronize( inPasteboard );
require_action( syncFlags&kPasteboardModified, PasteboardOutOfSync, err = badPasteboardSyncErr );