Their behavior towards you can be a dead give away that they are no longer "in love" with you. If you've dropped down on their priority list that's the first start.
A person becomes more distant, has less time to spend with you, when they are with you they act as though they're bored or on punishment and would rather be someplace else with anyone else, irritable/short tempered over the slightest things, is inconsiderate of your feelings, only does things to appease you or shut you up, and no longer talks in terms of future events involving you two. (Getaways for holiday weekends or upcoming shows)
Once their phone rings or someone else comes by there is a "new light" that comes into their eyes and they seem bubbly. Once the call ends or the person leaves and it's just the two of you again they go back to being sullen.
"Never love anyone who treats you like you're ordinary"
- Oscar Wilde
Odds are if you suspect someone doesn't love you it means they're not making feel all that "special". Maybe you chose the wrong mate for yourself.