You are responsible for playing the correct golf ball at all times, so it’s wise to put some kind of mark on it, perhaps with a Sharpie pen, to make it easier to confirm that it is yours. There is no obligation to do this under the Rules but if you're trying to figure out relief options for a ball which is visible but not retrievable a little distance away – perhaps stuck up a tree or deep in thick undergrowth – it can certainly help.
If you haven’t put an identification mark on your ball you should still be okay as, if you find one which is the same brand, model, number and condition as yours in an area where you expect it to be, that is usually good enough, unless a similar or identical ball is in the same area and there’s no way of knowing which is which.
If your ball is badly buried or hardly visible such that you can’t be sure it’s yours without further checks, you are allowed to lift it to take a closer look under Rule 7.3. But don't just wade in and pick it up as you must follow the procedure below to avoid incurring an unnecessary penalty.
First, you may only lift your ball if you are unable to identify it without doing so. If your identification mark is clearly visible on top, you may not lift it. Then, you must remember to mark the ball’s position before lifting it, using a tee peg or something similar. Finally, if there is mud or dirt on the ball, you may only clean it to the extent required to identify it, so take care here. Once identified as yours, you must then return the ball to its original position.
If you breach any of the above requirements you will get a one-stroke penalty, and furthermore, if you fail to replace the ball in the correct spot, the penalty will escalate to the general penalty, so two strokes in stroke play or loss of hole in match play for playing from a wrong place.
Finally, Rule 7.1b covers how to proceed when you need to identify a ball in sand, whether a bunker or other sand on the course. In this scenario, if you have to move sand to make a positive identification, you must then re-create the original lie in the sand before playing but are entitled to leave a small part of the ball visible if it had been completely covered originally. If you play without re-creating the lie, you get the general penalty.
