I think my cousin may be my half-sister?

I have always known my "father" wasn't my biological dad, well found out when I was 13. anyways, i kept digging, and found out there's a slight chance that my cousin (first cousin) (my mom's sister's daughter) may in fact be my half sister. i am broke. how can i find out the truth, in a cheap way. and dont say ask her mother, because her mom is coniving and lies. and the bio father is a loser who left the picture a long time ago. thanks.