The signs of infidelity, in what comes to psychology, are actually not very difficult to be detected.
The clearest ones are probably these:
a) The person does not talk about deep things with us, keeping it all outside of the personal, if possible, at all times, so that it is all sex, fun, and, if possible, nothing else;
b) The person does not invest in us: They do not observe anything about what colors match our organism better or what fabric, they do not see clothes and say we would look great on those, for instance, they do not observe anything about our makeup or perfume or clothes to the positive side, even though they can observe to the negative side;
c) We cannot count on them to make decisions regarding our own lives, like they seem to always be either useless or harmful when it comes to sharing thoughts in those regards. They seem to never really care about our future, even though they may sometimes mention something about a possible future together. The things they mention will be more abstract than material, so that they may say I keep on imagining us on a Country house and blah and blah, but they never mention their contribution, what they will be doing to get us there;
d) They seem to not listen to what we say to them when it comes to what they could do to improve the way they look or the way they do things. If they ever improve something because of us, they will never say it was because of us, so that they will never recognize our positive contribution to their lives, especially in public, rather the opposite. They may even call us names, give us bad nicknames and things like that;
e) They will not concentrate on us when there is plenty of time to be together, so that they will either be busy doing something else, and the more unfaithful they are, the more stupid the thing they will do during that time will be, so say a video game, or they will be with us but absent and the only time they seem to connect for real is during sex;
f) They will never be 100% present, like there will always be some part of their being that is not going to be with us, so that we could be driving, for instance, and, instead of helping us park, say, like going out and signaling or something, they will either ask to drive themselves or will be useless. A faithful person is together with us always, invariably, but an unfaithful one always keeps their mind somewhere else if it is not about sex. A faithful person will have a team on us, we will be one with them, but an unfaithful one will have a team on the others, so that that could be a mother, a brother, a friend or whoever else;
g) A faithful person does not exclude us ever from their leisure time, rather the contrary; they are able to cancel an event or adapt just to be with us. An unfaithful person will, instead, always find something that we cannot do with them, so say if we have fear of heights, they will be climbing and cannot take us because we have that fear;
h) A faithful person lives in function of what we like on them and from them. An unfaithful person lives in function of themselves and themselves only;
i) An unfaithful person will frequently make us feel bad when we are happy and celebrating life, so say they will say something looks bad on us or we did something really wrong on that occasion. A faithful person will simply be light and help us live happier at all times or most of the time;
j) An unfaithful person looks for reasons to fight, to go away from us. A faithful person looks for reasons to be together, to never leave our side.
k) A faithful person is always looking for ways to help or please us. An unfaithful person is always indifferent to our problems either most of the time or all the time;
l) A faithful person greets us first and feels happy that we are around, that is all visible and we feel welcome. An unfaithful person takes some time to greet us, will perhaps greet others before greeting us, and will have no special eyes because they are seeing us after a time away;
m) A faithful person will fear losing us and will have all care on earth with all they say or do, always monitoring how we feel, what we think, and etc. An unfaithful person cares about nothing. They may throw themselves on our personal space in our apartment, for instance, and even when we say that was our spot, they will still do it again without any consideration whatsoever; and
n) A faithful person stops everything to greet and welcome us. An unfaithful person may keep on giving attention to others when we arrive.