Hi,
Actually you really can't - and that is a big part of why some people dislike the hybrid idea.
If there is a hybrid in the genepool no matter how far back you can never gaurantee any resulting animal as pure ever again. You can say "It doesn't look like there are any borneo genes in it" but really is the best you can say.
That may be one reason why people are occasionally reluctant to deal with people known to create hybrids. You are totally at the mercy of their record keeping skills.
dr del