You think obsessively about the other person all the time. You think they're the most wonderful person you've ever met, you wonder what they see in you, and worry about losing them.
The Beginnings Of True Love
You think about having kids together, or being Mom to his kids
or Dad to her kids. You think about growing old together. You tell everyone
you know how wonderful he or she is. If he or she is happy, you're happy,
and you fret if he or she is not. You're best friends.
They call when they say they're going to call. They show up when they say they will. He or she is there for you.
If It's A Man
A man who's serious about you will want to make plans way into the future and see you on a regular basis. He'll want to see you on weekends, Saturday nights and holidays.
He will also spend money on you. If he's not well off, he may not spend a lot, but he'll do it with joy, and you'll feel a sense of generosity from him. If he never gives you any gifts, or always wants you to pay your own way, you can assume he hasn't learned to share or just doesn't want to share with you.
If It's A Woman
A woman who's serious will want to see you on a regular basis. She'll be available on weekends, Saturday nights and holidays.
She will cook for you (or try -- give her credit for trying), dress up for you, and also spend money on you. If she always wants you to pay for everything and she never offers to pay or to at least reciprocate in some way, by cooking dinner or buying tickets, or doing something for you, you can assume she hasn't learned to share or just doesn't want to share with you.
The real answer is actually simpler. The proof is in the pudding. True love should lead to commitment. The acid test of how you love each other is how your relationship is progressing. See "The Steps to Commitment"; this article is extremely important for anyone seeking a fulfilling, committed relationship.