There are two ways that people understand “true love”.
The one which most people think of the emotional aspect of true love: the passion and feeling of euphoria in spending time with your love. This is also referred to as “chemistry” between the two people.
The second aspect of true love is self-sacrifice. It is an act of the will to give yourself to your love, to sacrifice your own comfort for their sake. This is not emotional per se; it is an intentional act, not driven by passing, temporary emotions.
Now to your question: how can you know if you are truly in love?
In regards to the first aspect, you yourself should answer that; do you are do you not have chemistry with this person?
In regards to the second aspect, there are more questions to ask.
How much are you willing to give up for this person?
Can you let go of your own will and do things for the good of the other instead?
Are you interested in the relationship solely because you want to “get” something out of it, such as the emotional joy you feel or for the physical affections that come along with it? Or do you want to give yourself, to lead both you, her, and your future children to Heaven?
If you can give good answers to these questions, I would say you are on the road to being well prepared and committed for marriage.