The subject of "tell" is a direct application of the same subject from the game of poker. Mike Caro is consider the expert in this subject for poker and he has a book and a few videos for "tell" the body language of poker. In the game of professional football recently, some people is trying to develop "tell" on individual players and teams. Tell in the game of bridge is partially body language and partially habit in bidding or playing of hands. In many sporting competition coaches examine opponent films extensively to find their strengths and weaknesses.
But in the game of bridge there are very few discussions of the subject of "tell". So anyone who pay attention to this subject is going to have a significant advantage. It is most useful against the majority of players between intermediate level to advance level. For beginners their behavior is erratic and it is better off not taken as "tell". Since they have not been playing for too long, they do not develop enough "tell". They just do not know what to do in many situations. I have not play against the top layers enough to give an opinion. If anyone who have experience in the "tell" of top players, feel free to email me. I will give credit to anyone who provide me with information. "Tell" is most profitable against weak players who have been playing bridge for a long time. Consistent winners in club games are usually good in using "tell".
Here are some observations:-