It would seem that the suri phenotype is governed by only one gene, with two alleles, S and s, S being supposedly completely dominant over s. The resulting phenotypes for each combination would thus be:

• [ss]:
  
• [Ss]:
  
• [SS]:
  

• Huacaya
  
• Suri
  
• Suri
  

If this is true, it is then quite easy to check if a male suri is homozygous (i.e. [SS]) or heterozygous (i.e. [Ss]) for the S gene by mating him with female huacayas, as illustrated on the graph below. This of course also applies to a female suri, by mating her with male huacayas.

This means that, after only 5 matings with huacayas, all producing suris, one can be reasonably sure that the tested suri is homozygous [SS] as the probability is 96.88%.
After 7 such matings, the probability reaches 99.22%.
After 10 such matings, it reaches 99.90% !!!

For more information on that topic, refer to the following article: BAYCHELIER P. -- Suri and Huacaya: Two Alleles or Two Genes? --  Proceedings of the 2000 Australian Alpaca Association National Conference, Canberra, pages 79 to 85. Click here for a copy.