About 5% of the mainline 737/787 aircraft that fly AS639 have Starlink. A firm answer comes about 2 days before departure, once the aircraft is assigned.
See every Alaska Airlines aircraft on the fleet page.
Questions
How do I check if AS639 has Starlink?
Pick your travel date above. Alaska Airlines assigns the aircraft about 2 days before departure, and the answer then comes from that aircraft. Before that, you get the share of the aircraft flying AS639 that have Starlink.
How accurate is this?
We combine Alaska's own status data, public fleet data and the aircraft seen on each flight. Aircraft can be swapped before departure, so check again the day before you fly.