Try building a query based on looking for the PXE Proxy program. I can't remember if it will work for 8.8, but for 9.0 and later the PXE reps should be running a program called "LANDesk(R) PXE Proxy"
You may also be able to view the Inventory of a single computer you know has the PXE rep installed to get a better idea of how to construct your query to locate the others.