It's hard to talk about this boundary without using example and comparison. They don't give an exact picture, but they suggest and give you a sense of the reality.
If you say, "I love my dog," but look at an apartment that doesn't allow pets, you only liked the dog. Whoever said, "You can fall in love with a rich man as easily as a poor man," never loved. Whoever thinks love can be measured by a bathroom scale or diminished by age, never loved.
Love is like the answer to a riddle. Other answers may seem good or clever, but only this one answer is perfect.
Love is like the right job. The job inspires you and feels like play. You crave the work, you are passionate about it. It lifts you up and drives you to new levels.
Love is the color which connects with the deepest level of your being. It is the music which speaks to you.
When you love someone, you are totally yourself with them. Nothing can drive a wedge between you. Whatever life throws at you, you deal with together.
When you reach the boundary between like and love, you know you are entering another country. You are beyond newness and infatuation. You know what Shakespeare meant when he called love "an ever-fixed mark that looks on tempests and is never shaken."