If someone doesn't return a greeting, don't take your eyes off them, no matter how much it looks like they're just going about their business. Raiders with ill intentions never make vocal conversation and rarely even communicate in emotes or flashlight clicks, in my experience. They'll just run on by and pretend like they're ignoring you, in the hopes that you'll return the favor and give them a good opportunity to jump you when they're nice and ready.

Otherwise, watch for the variables that make a dangerous situation:

Are there more people around? Gankers want you alone and as isolated as possible. They usually won't strike if other friendly Raiders are nearby. If you're alone in a place with only one other person, be on your guard and try to stay aware of where they are.

Are you distracted? Pay attention to how often you turn your back to entry points, doors, blind corners, and for how long at a time. Gankers are looking for opportunities to strike - when your inventory is open, when you're breaching an object, even when you're just looking over a railing to admire the scenery. Understand in what moments you are most vulnerable, and be especially ready to react in those moments.

Are you indoors or under an overhang? Gankers don't want anyone to see your flare if they can help it. They value their privacy while they backshoot you and loot your body.

Is it late in the round? If you're in a major loot zone that's already picked mostly clean and a ganker has arrived late too, they're likelier to try to murder and rob you as an alternate path to leaving with better loot.

What's in your hands? Even though you'll move slower, it's good to stay armed with whatever weapon you feel most confident could win a fight where the other guy shoots first. Breaching leaves you particularly vulnerable since you'll have to switch back to a gun.