Tells are something you see more of in “movie” poker rather than real poker. Most poker guides (as the OP alludes) discourage tell based play unless you are playing with people you are very familiar with and have studied over time. Even then, it’s a bit of a crapshoot as opponents can use this against you (“did he scratch his temple because he has a poor hand or because he wants me to think he has a poor hand?”)

The one suggestion that I do find helpful is not to look at your cards as they are dealt but instead look at your opponents’ faces as they look at theirs. Sometimes their micro-expressions betray them before they regain composure.