how do i tell if a tea has a high fluoride content?

I would think laboratory testing would be the only way. Unless there is a DIY test somewhere, but you would still have to do it yourself and first you would have to do your water to see how much is in there so you know how much is released in the tea additional to the water. (Assuming you live in an urban area which fluoridates its water).

younger leaves grown at higher elevations and in more remote (less polluted) areas have less, so the higher quality teas should have less in general than mass market low grade plantation tea, but like like Uniquity said there’s no real way to know for sure.