How are open reading frames identified in a genome?

An open reading frame must begin with a start codon (ATG) and end with a stop codon (TAA, TAG, TGA). So it's mostly just a matter of looking for long stretches of DNA which begin ATG, run for at least a few hundred basepair, and end with a stop codon. Generally, only genuine open reading frames of real genes extend for significant distances; any sequence read out of frame will quickly hit a stop codon. This is because stop codons will appear quite frequently in DNA unless natural selection acts to prevent it from appearing in a region.