Why not do it in one find?
Go to layout ["Quote Item"(QuoteItem)]
Enter find mode
Set Field [ QuoteItem::quote_id;$record_id_variable ]
Set Field [ QuoteItem::quote_item_status; "X" ]
Set Error Capture [On]
Perform Find 
If [ Get(FoundCount) ]
Go to layout [ $layout_name ]
Show custom dialog ["Note"; "This quote item is already on a Purchase Order"]
Why not do a find for the records AND the 'X'.
If Get(FoundCount) >0, go to layout and exit the script.
After that check, do the find for the records alone and do your other stuff.
If this is all in the same script, in that order, you'll dump out of the script before you "do various other things" if any have the 'X'.
Sidebar question: Do you want to leave all of the records alone if ANY have the X? Or do you just want to omit the ones with the X from the found set before you "do various things" to the un-X-ed ones?
Well, the end goal is to omit those with an X from appearing in a portal on a different layout. Is this where a portal filter would come into play? Can you omit records in a portal is the value of a field is "x"?
Thanks to both of you for the input so far.
Is this where a portal filter would come into play?
Yes you will ahve to use a portal filtering technique if you plan on useing a portal. There should be lots of sample files out on the net regarding filtered portals. It may be easier to search and download those rather than us try to explain the techniques here.
You can also search this forum and find a number of threads that discuss the concept.
Try this one for starters (there may be better ones than this...look around):
but keep in mind that for the "omit" you are trying to do, you'll need your relationship to be something like:
Portaled Table Layout Table
ID#_fk = ID#_pk
Xfield ≠ globalXfield