Well it's pretty f*cking simple and obvious:
1) They don't play ANY of these mind games.
2) They don't cancel last minute, and they arrive on time. This shows interest and seriousness. If you wouldn't arrive late for a job interview, why would you for a date?
3) You don't have any doubts that they're talking to other people at the same time. After a few dates, these doubts should be completely gone. And I mean COMPLETELY.
4) You can FEEL their interest, and they express it. Without you having to guess, overthink, or play 21 questions.
5) You see each other every week for sure, and with time, 2-3 times per week. As time goes by, if you're seeing each other LESS often...they're not interested. Trust me.
6) They compliment you on things other than your appearance. Meaning they pay attention when you speak, and value your achievements and personality traits.
7) When they sleep with you, they always spend the night and never "disappear" in the morning AKA they are not using you for sex.
8) They are kind to friends and strangers alike. How a person treats others will say a lot about how they will treat you.
9) They prioritize spending time with you. You don't feel 12th on their list of things to do. I say top 3 is a necessity.
10) They care about your friends' and family's opinions. They want to be liked by them, and consequently they make sure to treat you properly in order to gain their trust and respect.
11) They're there even when you're not at your best. Even when you're dating bad sh*t can happen, and if they stick around and don't run away the first chance they get...KEEPER.