I went to PV in April and I noticed that even in the stores they were selling Alpaca (plated). I was telling people to look for 3 stamps (.925, Mexico and a manufactures number) but now the vendors stamp it on too. In Mazatlan, normally there is no problem with the stores, but in PV I guess it is a different story. I would say ask for a reputable store. In Mazatlan I could mention stores like Indio shop, Mr Indio, Michaels Gallery and others.
I hope this helps!