From what I know that the female folks are more perceptive intuitively via there psychic disposition to smell a rat from afar than the male counterparts. To this end, it won't be hard for you to know if your boyfriend has lost interest in you.
Interestingly, the signs would all be there for you to see. You don't need any third party to tell you that he is longer into you. Some of the major indicators are: he has suddenly become indifferent in reaching out to you; he no longer calls or text you as he used to. And when you call him, it seems as if you are pestering him.
Additionally, he no longer finds you fun to be with maybe he now prefers hanging out with boys instead of spending time with you. Plus, and if you're together, he no longer likes to do those things that makes you happy, for example, playing with you, calling pet names etc. He has stopped being romantic.
The truth is there are thousands of indicators that you can count on in determining if your guy is still interested in you or not.