1. Starflier or any under level enters field, now chances are they are gonna come right up to you and sit there while they send messages to their pirate buddies.
2. At this point there is no definite way to tell that he is scouting, but because he is close, not moving and looking right at you, you are suspicious. He can't possibly do anything to you so move away.
3. Here is the moment in which you will know for sure the true nature of this under levels presence. If he follows. If he does, fly in random directions, "oh look, I'm 30k about the plain, I'll fly 10k to the right and then straight down and to the right again". Pirates can't follow you if you take random directions like that.
4. Pirate will eventually get sick of trying to track you and will call his minion off.
5. you are safe.
Either we follow these instructions or we introduce a clause that states that "players under level 30 cannot be used for scouting. Vessels that are deemed to be scouting regardless of level are able to be destroyed". Now, yes I have thought about the abuse that this may lead to but by making the players follow these instructions we can prove beyond reasonable doubt that yes, they are scouting.
Now, I can hear your brains saying "but what if they want something and are following us for that reason". Well if they are asking for advice I'd still be cautious but if they are silent, run far far away. If they are asking for advice answer it for them while your cruising away from them? then if they keep following that act in itself proves that they are guilty.