How can molecular biologists identify genes in sequences of DNA?
The easiest way is to have a cDNA clone of the gene.
If you take total RNA out of an organism and reverse transcribe it into a cDNA and you sequence it, you'll have sequences of the exonic sequences of that specific gene.
THe gene on the DNA will contain introns too, but with the exonic sequences you can determine where your gene is on the entire sequence of DNA.
That is a pretty straight forward way. However, I think there are statistical programs that help identify promoter regions, 5` UTRs, ATG regions, etc.