Several options for hanging Christmas lights won't damage gutters. Using shingle clips avoids any hardware contact with them, and using plastic gutter clips minimizes the likelihood of scratching gutters. Remember not to rest a ladder, especially a metal one, against the gutters, and use a ladder stand-off device or stabilizer if you need to use an extension ladder. 

In the Christmas days of yore, people braved winter weather to staple or tack their Christmas lights onto their rooftops or nearby surfaces like the fascia boards. Problems quickly emerged with that approach. Damage to the roof, even small marks from staples, provides entryways for moisture and similar issues that can hasten roof rot, wear, and associated leaks. Plus, some homes had tough fiber cement boards or other materials where staples and screws didn’t work.

Yes, you can try removing and replacing each individual light bulb one at a time until you find the problem bulb—just be prepared for it to take a while. The bulb tester is worth the investment if you don’t have a lot of extra time to spare or need to test several strands of lights.

It's safe to hang Christmas lights from gutters with a few considerations. First, when installing the lights, follow safe ladder-use guidelines and avoid getting onto the roof. Second, for best results and the integrity of your lights, use clips to keep the bulbs away from inside the gutters where they could run into trouble with water or ice.