Several tests could be possible for the functionality of a DNA sequence. One would be to examine it to see if all of the features of a normal gene were present (promoter, intron, exon, start codon, stop codon, etc.). Another would be to clone the sequence into a plasmid and insert that plasmid in to a bacterium. Then tests could be run to see if the sequence was really functional by checking for the biochemical or genetic function expected from that sequence.
Work Step by Step
The best test of sequence functionality is to see it function, and the simplest way to do this is to put the sequence on a plasmid and insert that into a bacterium, testing for the actual function of the sequence.