How do you know your boyfriend kinda likes another girl?

Ok , so my friend since 3rd grade kinda likes my boyfriend bout im not so sure if my boyfriend likes her a lil , but i hav a feeling he does this is why, wen we wer in skool i saw my friend since 3rd grade flirting wif my bf but if he really didnt lik her would let her flirt wif him in the first place!!!!! n in my mind it keeps say o he kinda likes her but im just not very sure i mean he talks 2 her more then he talks 2 me (just a lil) , i feel sometimes that it can b becuz im geting a lil jealous , he likes her just a very lil or just all of the above , i just cant put my finger on it.PLz plz plz help me!!!!!!!