I vs D refers to two alleles (forms) of the gene for ACE (ANGIOTENSIN
CONVERTING ENZYME). In the work quoted below, it is tested by actually
testing the DNA directly, using PCR (polymerase chain reaction). I and
D have different gene lengths, so they give PCR products of different
lengths. (I didn't look at how they do it, but that would seem the
likely simple way.)

The following abstract is at
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=Abstra
ctPlus&list_uids=17487330&query_hl=1&itool=pubmed_docsum

1: Rev Diabet Stud. 2006 Summer;3(2):76-81. Epub 2006 Aug 10.
Links
The impact of the Angiotensin-converting enzyme insertion/deletion
polymorphism on severe hypoglycemia in type 2 diabetes.
Freathy RM, Lonnen KF, Steele AM, Minton JA, Frayling TM, Hattersley
AT, Macleod KM.

Institute of Biomedical and Clinical Science, Peninsula Medical
School, Exeter, United Kingdom. The first two authors contributed
equally to this work.

The insertion/deletion (I/D) polymorphism of the
angiotensin-converting enzyme gene (ACE) is associated with altered
serum ACE activity. Raised ACE levels and the ACE DD genotype are
associated with a 3.2 to 6.8-fold increased risk of severe
hypoglycemia in type 1 diabetes. This relationship has not been
assessed in type 2 diabetes. We aimed to test for association of the
ACE I/D polymorphism with severe hypoglycemia in type 2 diabetes.
Patients with type 2 diabetes (n = 308), treated with insulin (n =
124) or sulphonylureas (n = 184), were classified according to whether
or not they had previously experienced severe hypoglycemia. Samples of
DNA were genotyped for the ACE I/D polymorphism using two alternative
polymerase chain reactions to prevent mistyping due to preferential
amplification of the D allele. Overall, 12% of patients had previously
experienced one or more episodes of severe hypoglycemia. This
proportion did not differ between genotype groups (odds ratio (95%
confidence limits) for carriers of D allele relative to II
homozygotes: 0.79 (0.35-1.78)). This study found no evidence for
association of the ACE I/D polymorphism with severe hypoglycemia
frequency in patients with type 2 diabetes. However, we cannot rule
out a smaller effect (odds ratio </= 1.78). Our results suggest that
any effect of ACE genotype on severe hypoglycemia risk in type 2
patients is likely to be smaller than that seen in type 1 diabetes. We
recommend future larger-scale studies.

PMID: 17487330 [PubMed - in process]

I do not know if alternative assays are available for routine lab use.

If you want to browse further... I put the following into a PubMed
search:
ANGIOTENSIN CONVERTING ENZYME genotype DD
A number of titles look like they might be worth checking.

bob