If somebody is looking at you, playing "tourist or local", they will probably be able to tell. Be it fashion, physical features (size, being Sicilian-American in Norway or vice versa), presence of a day bag, or just the way you are walking. If someone cares enough to pay attention to you, they will likely figure it out.
BUT I have a goal not to stand out as an obvious tourist. This is for my own protection (obvious tourist reads as easy mark for thieves), as well as my own comfort. I just feel more confident when I don't seem ridiculously out of place. This falls into two categories - physical appearance and behavior.
Clothes, etc: my guidelines FOR ME in Europe are no sneakers (I love the trendy fashion-blog sneaker look and see plenty of Europeans wear this, but I can't pull it off and look chic, I just look frumpy), no shorts in cities, no "travel clothes", no "athletic wear", no logos or sayings on shirts, no backpacks (purse works fine) and more classic style in general. I take some inspiration from what is in international stores/lines like H&M or Zara or Mango or Benneton. I'm in my 30s, I'm sure there are different brands more appropriate to people significantly younger or older. I also wear basic, minimal makeup (takes 5 minutes) and I bring a scarf or two. A typical outfit might be skinny jeans, black blouse, scarf, ballet flats, gray purse. A really bad "tourist" outfit might be ill-fitting travel cargo shorts, mickey mouse t-shirt, college football logo hat, sport sandals with white socks, backpack with guidebook hanging out, selfie stick.
Behavior - talk quietly, walk with purpose, if stopping on a sidewalk, move to the side, know where I'm going (i.e. not walk with the map in front of me!), know local customs and don't complain about them (ice in drinks, espresso vs. brewed coffee, stores closed for siesta, "slow" restaurant service), know a few words of the language and use them appropriately. Most importantly: carry self with confidence. Projecting discomfort makes you a target AND worsens your trip - you are less likely to seek new things (the whole reason you're traveling!) if you feel constantly out of place. So do what you can to leave discomfort at home - for me, it's tons of research so I know what I'm doing once I hit the ground.