My female stray mane coon might be pregnant... how do i know for sure?

My female maine coon stray cat might be pregnant because she is very bonny and skinny every where but her belly is a bit big and sometimes in movement... her nipples are not swollen or very pink... she was starving when we found her and ate allot... she then puked and we feed her in smaller amounts now...but she just gets hungrier and hungrier....she never lies down on her stomach,,, only on her side.. and for a stray cat she is very affectionate..but we cant bring her to a vet because our parents don't know about her and wont let us get a new pet... she is kept inside all day though... help ASAP!!!