How can i find out if im adopted and im under age and i cant ask my parents?

i was looking through some old files whe i was organizing my moms office for her and i came accross some thing that made me wader if i was adopted it was a letter that just said please tell natasha about me. also i look nothing like my brothers who look so much alike or my sisters who when you look at there baby photos you cant tell whose who. so can anybody tell me how to find out if i am adopted?