By seeing a constant high-energy radar signal, which would distinguish it from surveillance radar.

That's my point. Maybe a notification protocol? Seems to be used as a warning sometimes..

Others have already commented, but normal surveillance radar is like a spotlight from the tower. It goes around and scans the area. If they determine that you are important, the radar can "lock on" to you which means that it will no longer scan around but would shine the light on you constantly.

If they want to shoot at you, they turn on another even stronger more focused light to improve accuracy.

Military today have ability to detect enemy targetting radars and when they lock on so having such a thing in SciFi does not seem strange to me.

Ah sorry I misunderstood your comment. I thought you wanted to say it was obvious for the target to know. Haha yes a notification protocol would be a weird thing to have. It would be gamed the millisecond things went beyond friendly warnings.

The same way they do now: by detecting whatever the attacker is using to sense and target them. Today, that's usually radar. In sci-fi, maybe it's tachyon beams or something.

Aside from detecting any guidance signal the trajectory changing towards them is a giveaway that it is homing and not just a dumb projectile that they are in the path of.