You can compare to the breed standard, listed here:
http://www.lhasaapso.org/

Of course, if the main reason you think she is not a purebred is size, well, that is just a "fault." Bichons are supposed to be 9 to 11 inches at the shoulder, but there ARE both larger and smaller purebreds; they just aren't able to go into show.

They work quite well for love!