RE: What are the signs?
Bottom line is, if you can't hit where you desire to hit, you need to get your bow shooting to hit that spot.
90% of the misses on game I bet are from nerves, not bow tune. If you can't get your broadheads hitting 4" groups at 30 yards, then keep your shooting to 20yards. And if can't get your BH's hitting 3" groups at 20 yards, might want to have it looked at.
Most of the folks on this page, are super anal. And sadly I am one of them. We desire perfect flight. I desire my hunting bow to be able to easily hit a deer vitals all the way to 60 yards. If I am not hitting 1" groups at 20 yards, I am bothered and will tinker and tinker.
I know from experience, most new bowhunters desire a perfect tune right from the get go. And from what I see, 90% of the problem is not the bow, but the guy behind it. Get a bow tuned by all means by a good pro shop, but then practice and practice.