I'm assuming you mean "photo period" instead of "pheno"? A phenotype is the characteristics of a specific organism. I just wanted to explain that in case you confused the terms and that wasn't a typo.
That said...An auto will usually start flowering within 30 days so the holding out longer than that would help you figure it out. The only other way I can think to get a faster answer would be to take a clone cutting from the plant and then put that clone into a 12/12 light schedule to see if it starts flowering. The problem with that second method, though, is it will stress the plant and slow it's growth a bit. So, if it is an auto, it can reduce the amount it can grow during its life cycle. Hope that helps!