A good sequencing reaction will produce between 300-900 base pairs of useable sequence. You should receive your sequencing results as a trace file (.ab1) which graphically depicts the sequence as a series of colored peaks corresponding to one of the four nucleotide bases. This is an example of a trace file from a high-quality portion of a sequencing reaction:
Sequence near the beginning or end of a sequencing reaction is often unreliable. Although your sequencing results may indicate bases at specific locations, by looking at the trace file, you will see that these base calls are unreliable. This is an example of a trace file from a portion of a sequencing reaction with high background: