It varies for different people, really..Generally, you know you must REALLY like someone (maybe even love them) if they are always on your mind..If just thinking about them puts a broad smile on your face..If every decision you make, you consider how it could benefit or harm your relationship..Maybe thinking about them gives you a happy, floaty feeling that makes it to where you cant stop shaking..If you can easily picture yourself being with them, and perhaps nobody else..^^
Love can be complicated, and nearly impossible to put in words.