Through observation and experience.
  1. After a long night of drinking, they'll leave a pain reliever and a glass of water on your nightstand.
  2. They'll give you a ride to the airport, train station, or bus stop without a question.
  3. They'll wait for you to safely enter your home before driving away.
  4. They will hear something potentially offensive/insulting and air on the side of your not meaning to be so...
    Suggested by @john
  5. They call you to wish you a happy birthday and find a way to make it special even if you didn't ask.
  6. When grocery shopping, they'll think of an item you're running low on and pick it up for you. They'll say, "I thought of you" and you both hug it out in the kitchen.
  7. Before pouring a glass of water for themselves, they'll pour a glass of water for you.
  8. They will pet sit and water your plants when you're out of town.
  9. If it's a "cash only" place and you forgot to bring cash they'll spot you. You'll feel embarrassed.
  10. They'll kiss you when you're sick.
  11. They'll call you out on your BS without being mean spirited.
  12. You can tell by the way they look at you.
  13. You can feel it and they can feel it too.
  14. Love is 24/7.
  15. Love doesn't quit.