Hi, if you are talking about Sanger sequencing (and the like) you need to extract genomic DNA, amplify a portion of the gene and clone it. Then you should transform competent bacteria and screen some of the resulting colonies. If the sample is heterozygote for the gene, you will have two different alleles in your sequencing results; otherwise, you will systematically get the same result.
If you do not clone your PCR amplicon to a vector, a crude way to assess whether you have a heterozygote sample is by evaluating the electropherogram that is normally attached to the sequencing results. With the right experimental design, you should be able to see double peaks associated with the different alleles.