IF you have a fruit fly with long wings and you are not sure whether the fly is LL or Ll?

Test crosses are always with a zygous recessive, so you will be breeding with a fly that has a genotype ll. If all the offspring has long wings, then the fly is most likely zygous dominant, but that doesn't mean that there isn't a chance that he is heterozygous. If there are short wings, then we know that it was heterozygous.