Just use your brain. If it looks like nobody else is around or could've possibly hit it there from where the groups around you are, then feel free to take it. If there's even a tiny chance that someone from the group on the box two holes over could have hit it there, then just leave it. It's only a golf ball after all.
Recap of an actual conversation I've had with someone who drove into the fairway of the hole I was on and picked up my ball from smack dab in the middle of the fairway.
Recap of the conversation once I reached their location:
Me: "Hey, you picked up my ball from the middle of the fairway."
Thief: "I did???? What type were you playing?"
Me: "Taylormade TP Black."
Thief: "What number were you playing?"
Thief: "Fine. You can have it, even though I don't think it's really yours."
Never before in my life have I wanted to hit somebody with a golf club so bad. Over something so stupid as a two dollar golf ball no less.
I can't be the only one who thinks should be perfectly legal to hit someone if they've stolen your golf ball right in front of you then proceed to lie/play dumb once you call them out on it. One could probably get out of the assault charges anyway because Judges and Lawyers play golf all the time, and I'm sure they hate having their ball stolen.