Yes, look at their family. If someone has unattached ears, and one of their parents has attached, they carry a recessive gene. As you go back, you can't be dead sure, but grandparents can also give you those sort of clues.
Mate them with someone with the recessive phenotype (hh). If all offspring have the dominant phenotype, then the individual is homozygous dominant (ie, HH), and the offspring will be heterozygous (Hh). But if some offspring have the recessive phenotype (hh), then the original individual is heterozygous (Hh).