Gun signs might indicate a predisposition to want to shoot folks.
There's that, of course.
One of those "Protected by Smith & Wesson" or "Forget the dog, Beware of Owner" say simply one thing -
"Do your homework and case my house for awhile. If you wait until I'm not home, you'll have all sorts of good stuff that's worth a lot of cash on the street. It will be worth your time, I promise"
Then again, there's always one that was suggested on another forum:
"CLAYMORE'S ATTACHED TO ALL DOORS AND WINDOWS....OPEN AT YOUR OWN RISK....OWNER DOES NOT HAVE LIABILITY INSURANCE...."