DNA and RNA are both made up of nucleotide. There are two major difference between DNA and RNA. In DNA deoxyribose sugar is present and in RNA ribose sugar is present.
The other difference is in DNA thymine nucleotide is present which makes complementary base pairing with adenine but in RNA thymine is replaced by uracil.
Therefore testing for the presence of thymine is most appropriate in this situation to find out whether a nucleic acid sample is composed of RNA or DNA.
