Generally when someone is romantically/sexually interested in someone there is a certain amout of flirting that takes place, subtle touching of the back, waist, leg, lingering eye contact, compliments on clothing, suggestions of spending one on one time together.
Also observe the topic of discussions. If someone is interested in you they won't talk about date plans with other people nor encourage you to talk about anyone you may be dating.
Some people are so introverted they want the other person to give them a "sign" they will not be rejected. This could lead two people to be waiting on each other to make a move. Everyone hates rejection.
At some point one of them gives up and puts the other in the "friend zone" by talking about dating other folks.
On the other hand it could be the only thing this person is interested in is a platonic friendship or a brother/sister relationship. You generally know that's the case when they pour their heart out to you about exes or ask you for dating advice.
Having said that mature men and women who know what they want seldom come off as ambiguous when it comes to their romantic interest.