This question comes from Reddit and the short answer is, “You can’t.” At least not entirely.
The longer answer is that women that are interested in you will be friendly, BUT TYPICALLY, they will also do other things to show interest as well. The more indicators of interest they give you, the more likely it is that they like you.
What are examples of indicators of interest?
Seeming to be friendlier with you than other people. Finding ways to be around you. Touching you. Responding to you touching them with touching you. Asking you questions about yourself. Laughing at your not particularly funny jokes. Asks for your name. Gives you meaningful compliments. Holding eye contact. Looking down when you hold eye contact with her. Staring at you. Saying things that make it clear she’s trying to impress you. Starting or reinitiating conversations with you. Noting that she’s single.
What makes this complicated is that SOME WOMEN who are interested in you will give you few or even no indicators of interest.
Other women that are just being manipulative or that want validation via male attention may give you a number of indicators of interest without liking you.
However most of the time, the more indicators of interest a woman gives you, the more certain you can be that she likes you. Sometimes, especially with experience, you can get enough to almost know for sure, but you’re often just flying blind unless the IOI’s start to stack.
All I can tell you is that you need to get a thick skin, accept that you will be frequently rejected and feel your way along.