Promoter sites specifically bind RNA polymerase and initiate transcription.
There are certain characteristics common to promoter sites which we can use
in trying to locate them. For example in bacteria, on the 
(upstream)
side of the start of transcription the sequence TATAAT (known as the Pribnow
box) is found at about -10 bases, where -1 denotes the base before transcription
and +1 is the first base transcribed. Note that there is no zero-position, and
slight variants from TATAAT exist.
For eukaryotes, a major characteristic sequence in locating promoter sites is TATA, known as the TATA-box, found at about -25. There is another typical sequence at about -75, and in addition to these there are enhancer sequences which can be found up to a couple of kilobases distant on either side of the promoter site.