Not much. That's why you send someone to actually take a look up close, and see if that's actually the case or not. OTOH, both pilots dozing off or becoming distracted - now that tends to happen, too. In that case, "knock knock, who's there? German Airforce" tends to resolve the situation fairly quickly and without further incident*, as seen here.
(Other than "a wild mountain of paperwork appears")
Could've been cabin preassure loss and all of them are passed out all dead just as much as hijacking.
Shooting down can be just as achieved with surface to air missiles.
We send jets to establish contact and use short range and if needed LOS communication to figure out what is going.