It would depend on the person doing the observing I guess, some people are definately more aware of their surroundings, but then they would also have to be willing to look for certain tells and recognize them. (THis is all theoretical).

On the other hand, it would depend on how conifdent and safe the person or couple is in different situations. Also very much the 'mood' the person is in, which I have found is very important as to the signals a person will send out. The more of a 'mood' positive or negative, tends to have larger tells.