Cathay Pacfic, YVR -> JFK.
...plus Philippine Airlines to LAS as already mentioned.
Originally Posted by
Delta20
How do I otherwise tell before booking it if a flight goes through pre-clearance?
With a few rare exceptions mentioned, you need to pretty much always assume your flight goes through pre-clearance.
Another option is to drive to BLI airport and feed into Alaska's network from there, or fly Allegiant Air. (Delta used to serve BLI, but they may have pulled out...)