I believe you first have to ask yourself why you have any doubts.
Everyone has their own idea of what "real love" is suppose to look like, encompass, or feel like. Some folks feel if their mate loves them they'd profess it in words constantly, or shower them with gifts, outings, and surprises, and others want intimate conversations, affection, and passionate sex. Very often if someone is NOT loving us or expressing their love for us the way (we) want or believe love should look like we assume they're not into us.
Sometimes it's a matter of "bait & switch".
In the beginning of a (new) relationship BOTH people are bending over backwards to impress one another. They agree on everything, they're talking for hours, laughing, having steamy sex, and planning romantic outings. Cards, flowers, and gifts are given to one another "just because".
After there is an (emotional investment) or "commitment" the (infatuation phase) comes to an end and each person reveals their (natural) or authentic selves. Quite a few people say this is to be "expected" or normal.
However if you're the naturally romantic, affection, and sensual type and your partner starts to seem less so after awhile you begin to have doubts that they are "into you" as much. While in reality this is their normal pattern.
Simply put you both have (different) expectations of what love should look like and feel like. Your mate may feel the love between you has been established and you don't need to do all the things you used to anymore.
Having said that sometimes a person really does "distance" them self prior to breaking up with them. It becomes clear that your happiness is not on their list of priorities. Conversations become very short, they're easily agitated, romance and passionate outings give way to watching TV and going to sleep. There is a complete lack of enthusiasm regard future events or plans. You don't feel like you can be your complete yourself around them. These are all signs that you're unhappy with the way things are.
Communication is always touted as being the "solution". However if for example you told them they never buy you flowers anymore or they don't initiate sex and the next day they gave you flowers or initiated sex...A part of you would resent the fact that you had to (ask) for something (you considered natural) for one to do who is "in love". They only did it to "appease" you. It doesn't make you happy.