Is there an inexpensive test (IE: perhaps a form of litmus paper?) for keeping tabs on oral pH at home?

My initial research into the effect of oral pH on health I came across an article that suggests that using fluoride products in an acidic environment has many negative effects on health and that 32% of American children suffer from fluorosis, mostly from naturally occurring fluoride in groundwater!

Given that information, what might be the best way to go about testing one's oral pH?