Just like hCG is used to test if someone is pregnant, it is possible to determine if someone is in love based on their brain waves, body chemistry, and other characteristics. This is quite expensive, and generally not recommended, especially since love is unnecessary for a happy and successful romantic/marital relationship, in fact it may get in the way.
Of course you can. The answer is obvious. Your heart will tell you. It's hard to describe. You get this tingling feeling when you're around that person, you just want be with them, and you feel comfortable around them. Most people don't know what they are feeling when they are feeling it.
Yes, you can tell when you truly love someone. As a guy, it's different than just looking at her boobs and butt as she walks by. It's being attracted to who she is on the inside along with the outside, having true feelings for her from deep in the heart and hoping she's doing ok. Strangely, it's almost a sad feeling as well as a good feeling, one that really can't be explained with words. If you really care about her and aren't just horny about her, you'll know it from the heart without having to question if you really love her or not.
The emotion we call love is just like any other emotion as it is created by a combination of chemicals in your brain it is when you experience a particular mixture of the chemicals that you begin to feel emotions just as you know when you are happy or sad you also know when you feel love.
How do you literally know that you love someone? The answer is you can't. You can think, but you can never be certain. You could say that you love your parents, but is that entirely true? What would you do for them? Have you harassed them? What have you done for them? What could you still do for them? These are all questions that tie into this.
Personally, I do not believe in soul mates, or people who are meant to be. I believe in love in the way that it can come from anyone. Nobody is meant for each other, but when you're with someone for long enough I believe that you can fall in love. We all have different levels of love. What I might consider love, you may consider to just be liking someone. So honestly, I don't think you can always tell, because this debate brings up another question... What is love? There is no definition. You can't pin point something and simply say "that is love", because no one knows.