1. Facial expressions – their features soften, they express interest through raised brows or dilated pupils.
You know that look – the one when someone’s face somehow swells with emotion without really changing expression. Someone who loves you can control whether or not they say it, but it’s unlikely that they’ll be able to entirely control their subconscious responses (such as continuously glancing at your lips, a general enthusiasm when you speak).
2. Gestures – they physically open their body to you or try to “take care” of you.
They face you directly, acknowledge you consistently, and do their best to actually keep their heart centers as close to you as possible (think of hugging someone you care about – your core meets theirs). Gestures can also include grabbing your coat for you, or plucking an errant hair off your shoulder – things that signify a desire to take care of you.
3. Paralinguistics – their voice shifts in loudness, inflection or pitch.
They either raise their voice or lower it – they either strengthen their tone in trying to make you feel secure, or soften it in trying to make you feel safe. Regardless, any shift in paralinguistics signifies a desire to win over, impress or otherwise convince someone of something (in this case, that you should also be attracted to them).
4. Posture – they hold themselves upright, face their feet toward you, etc.
Body language is greatly comprised of posture, which is the way our bodies communicate what we’re really thinking or feeling. Someone who loves you will want to hold themselves in a certain manner in your presence, either to appear more attractive or to seem more interested. Look particularly for how they sit, and how they cross their arms and legs (arms protect or close the heart, widening the legs indicate a desire to seem dominant).
5. Proxemics – they come into your personal space.
The amount of distance appropriate between two people varies by circumstance and culture, but most of the time, any casual interaction will occur at around 18 inches to four feet of distance away from one another. If someone is decidedly within your personal space – leaning in to whisper to you, sitting closely, holding your side or arm as you walk to protect you – they’re into you.
6. Eye gaze – they maintain eye contact, and their attention is strategically drawn to you.
The eyes don’t lie, and people usually can’t fake a connection in their eyes, even if they’re faking it in their bodies. You find their attention drawn to you when you walk into a room, or while you’re around a group of people. They pay attention to the little details about you, and repeatedly glance at your lips, arms, chest, hands etc. If your relationship is sexual, they maintain eye contact.
7. Haptics – they physically touch you, and often.
Touch is a powerful communicator, and the way someone touches you says a lot about how they really feel. Just as infants need skin-on-skin contact to survive, touch is how we communicate comfort, affection, attraction, security, and so on. Any excuse to graze past you or go in for another hug tends to mean they’re feeling something toward you, and are literally reaching for more.
8. Appearance – the colors they wear can tell you a lot about how they want to make you feel about them.
Other than basic color psych (red = passion; black = reservedness) the more they go out of their way to groom and dress themselves before they see you – the more conscious they are of how they appear – the more they’re into you.
9. Artifacts – the objects and images that they choose say a lot about who they are.
Not to get incredibly basic here, but the emojis they use, the photos they share with you, the objects they share with you about their lives (an important piece of jewelry, a guitar, a book) say a lot about who they are and how they want you to perceive them. Someone who loves you will want you to see them for a slightly aggrandized version of who they really are. 
