First, you should not look at words, but at deeds. For example, a friend is constantly in other company, or with his significant other. And he tells you that he wants to meet you most of all. If in fact nothing happens, then this is not a true friend. Such a friend can leave at any time.
Conversely, if you see that the person is constantly around, puts off other things, but not you, then this is a faithful friend. He really wants to be there for you, and will be there for you in a time of need.
Second. The person remembers a lot about you. Remembers your favorite food, a movie. Remembers your jokes, secrets and so on. We are built so that we remember what is important, and what is not important to us, it quickly flies out of your head. And if you see a person remember a little thing you said a year ago, that's a loyal friend.
It means that the person considers you an important part of his life. That's right, because there's a reason he remembers a large number of little things. And if a person forgets even the important things, then I would begin to doubt your loyalty. Important things, it is not always the date of birth, it is rather some secret or something personal.
Also, an important point in a friend's loyalty is to the detriment of your interests when you really need it. For example, a friend wants to watch an important soccer game, but you call him to go out. It is adequate to refuse. But perhaps you are going away for a long time, and this is the last opportunity. Then the faithful friend will miss the match.
Or, for example, you need money very urgently, and the friend does not have much. The faithful friend will cut himself some slack, but he will help if it is really necessary. That is, he will break off the last bun if you have nothing.