The most obvious sign is that your hookup partner will tell you straight away that she/he wants to make the current relationship to a serious one. But if she/he hasn't fully confessed the feelings about you, here are some signals that you can tell if your adult friends want to make things serious：
Introduced you to their friends. As we have talked about for casual dating, introduce friends or family to casual hookup partners are too serious to do that. Therefore, when your hookup partners introduce you to their friends, or ask you to meet them, which definitely means she/he wants a serious dating with you.
They want to meet your friend. Or, if they have already met your friends, they will ask you about them. Just like what we mentioned above, for a casual relationship, there is no need to know each other's friends and family. There is no chance to meet them, so, why bother? But when they want a serious dating, this is a prove that you two are no long in adult hookup dating.
They delete their online hookup apps. As we all know, online dating apps are designed to help singles to meet suitable partners or have casual encounters. If your adult friends delete their dating sites, it means they have found their love. And these dating apps are useless.
They remember the little things you mentioned and mentioned them in later conversations. Only people who care about you will remember everything you have talked. Love makes people pay attention to details and small things. If you have had serious relationship before, you should be familiar with that. This is how couple do for each other.
They have done many good things for you. For example, when you are sick, she will bring you soup, or bring you some snacks she made the other night, because she remembers you said you like to each them when you are watch movies. They like to stare at your eyes eagerly, no matter you are talking to them or not.
They start to contact you for anything. They like to share their feelings at that time. When anything up, they would like to text you or even call you. When they are upset, you are the first one or even the only one person they want to meet up. All these signs say you are in their hearts.
You may have an intuition that your friend finders want more. If you feel it, just say so. In a friendly and compassionate way, ask her/him if your current pure hookup relationship makes her/him uneasy or uncomfortable, or if she/he feels a stronger feeling. If she/he admits, it is best to end this casual hookup relationship immediately, so as not to hurt their feelings and make things more complicated. Or, if you have the same feelings, then why not try to have a serious date with them? Maybe he/she is your soul mate.