Personally, whenever I hear the "puff" sound of warping ships, I look at scanner and check them out. If they're cops I just make sure I'm the closest ship to the target than they are. (This is part of the reason why I don't listen to any music when gaming too... cuz I need to hear gaming sound cues).
And if I see one of those dots moving close to me I just stop firing and move away to another spot.
And if the cops are bringing my target's hull down to under 10%, I boost ahead of the cop and make sure to get in the final kill shot or else I get no money
So far, I haven't had a "friendly fire" incident at all doing this.
ps: There should be a way for us to lodge a complaint against the cop that accidentally hits me though. It's rather unfair a stray shot from a player gets a fine, but if they accidentally shoot us, nothing happens...