You should know the Thymine pairs with Adenine and that Cytosine pairs with Guanine. If you know what one of the Letters are (A,T,C, or G), then you can easily figure out the other letter. A trick is that Adenine pairs with Thymine and they are straight figures like the letter "A" and the letter "T". Guanine and Cytosine have the letters "C" and "G" and both of them are curvy and not straight. This helps you identify the other nucleotide if you know one of them.
