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.
At your LANDesk Console look under "Configuration -> PXE Representatives"
Since the core was upgraded the Cnfiguration -> PXE Representatives might not show older ones.
Best bet is to make a query like Kevin said. Only I would look under OS --> Services "Where Name is LIKE PXE"
Thank you to everyone who offered suggestions. The query for PXE was the best way to find them. I found it under "LANDesk Management / PXE Holding Queue". It did not list the new PXE reps, and only the old ones. (the new ones can be found using the console configuration menu).