Imagine that you were able to see a nucleotide under a very special microscope. as you scan the nucleotide you see a u nitrogen base. without seeing any other part of the nucleotide you know that

DNA and RNA are the nucleic acids made of large number of repeating sub-units called nucleotides. Each nucleotide consists of a sugar, a phosphate and a nitrogenous base. There are four types of nitrogenous bases such as adenine (A), guanine (G), cytosine (C) and thymine (T) in DNA.  Whereas, in RNA, base thymine is absent and it is replaced by uracil (U). Each base makes one nucleotide with sugar and phosphate.

While scanning the nucleotide one see a u nitrogen base, it means this nucleotide is a part of RNA because nitrogenous base uracil (u) is only present in RNA.  

there’s no image attached but i can if you want: )