Except for a few cases in which you can have proof someone is saying the truth, can you actually ever know if someone is being honest when they say things like "I love you", "this is nice" or "you look great"? Or do you simply have to assume the person is saying the truth because you feel like they're being genuine?
You can't ever truly know what someone is thinking. You have to engage in some form of inductive reasoning and make a guess. Of course you can be extremely certain based on previous behavior and interactions with this person, but at the end of the day they could be lying.
This is being kind of picky though, people have told me stuff that I could bet my life on, but i can't ever truly know since you can't know what is going on inside the persons head.
In general, you can and often do know when someone is telling the truth. Suppose someone says that it's raining. If you already knew that it is, you also know that they are telling the truth. In general, if someone says that p, and you know that p, you can effortlessly infer that they are telling the truth.
The OP asks:
Or do you simply have to assume the person is saying the truth because you feel like they're being genuine?
The real test is not the feeling one has when someone is speaking the truth, but the feeling one has when that person is lying. Although the lies may seem trivial, they put on a false show. When taken to an extreme that can be described as life-denying. What is life-denying can be described as evil.
M. Scott Peck, a psychotherapist who linked lying, narcissism and evil claimed that the best way to tell if someone is lying, that is, someone who is hiding their uniqueness to put on a show to look good, is to notice a feeling of "revulsion" in oneself toward that person: (page 65)
The feeling that a healthy person often experiences in a relationship with an evil one is revulsion. The feeling of revulsion may be almost instant if the evil encountered is blatant. If the evil is more subtle, the revulsion may develop only gradually as the relationship with the evil one slowly deepens.
The feeling of revulsion can be extremely useful to a therapist. It can be a diagnostic tool par excellence. It can signify more truly and rapidly than anything else that the therapist is in the presence of an evil human being.
As the OP suggests, one can best tell when one is hearing lies by how one feels. It is not only the therapist's best diagnostic tool, but also anyone else's.
The linking of lying with evil may seem too strong, but this is what raises the philosophical challenge underlying this question. For Peck evil (lying, narcissism) is a therapeutic concern that is very real. For the philosopher there is an ontological concern. Does evil actually exist and how should one use that word?
Todd Calder describes the various philosophical approaches to evil today as either "evil-skepticism" or "evil-revivalism".
Evil-skeptics believe we should abandon the concept of evil. On this view we can more accurately, and less perniciously, understand and describe morally despicable actions, characters, and events using more pedestrian moral concepts such as badness and wrongdoing. By contrast, evil-revivalists believe that the concept of evil has a place in our moral and political thinking and discourse. On this view, the concept of evil should be revived, not abandoned....
Peck promoted in contrast to both of these positions an evil that was real and linked that back to hiding one's uniqueness behind lies.
Calder, Todd, "The Concept of Evil", The Stanford Encyclopedia of Philosophy (Fall 2018 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/fall2018/entries/concept-evil/.
Peck. M. S. People of the lie: the hope for healing human evil. (1985) Retrieved on August 16, 2019, from Internet Archive at https://archive.org/details/peopleoflieh00peck