Assuming that we are talking about fruit flies, then you should be able to determine this. Gray bodied flies would be the dominant and ebony bodied flies would be recessive. So, the results can only be a homozygous dominant fly (GG) or heterozygous fly (Gg). If you cross that fly with a ebony bodied fly (gg) and there is a result of some ebony bodied flies, then you know the original fly was heterozygous. If you result in all gray flies, then it was dominant.