You send the text messages, you make the phone calls and you put in the effort to see one another throughout the week. If you don't, several days could go by without exchanging a single word.
If you're the one making most of the plans to spend quality time together and go out of your way to show the other person how much he or she means to you, chances are you're in a one-sided love affair. If you always have to be the first one to make contact, you should get out now.