How do you tell if flouride is making your teeth stronger?

Time: If other factors in your mouth remain the same, time can tell if Fluoride is strengthening teeth, as you will be less likely to develop tooth decay. If you have frequent Fluoride visits as a non-surgical option (instead of fillings), you may see the reversed cavity if this has not invaded though enamel layer. The effect on tooth is cumulative, so consistent, careful applications are needed.

Marginally stronger : Its a real myth that toothpaste with Fluoride makes teeth, especially adult teeth stronger. Sure there are studies that show weakened pre-decayed areas can slightly remineralize, but overall it's just a marketing tool. Now, if you are talking about children swallowing the stuff, (its toxic and not recommended) the Fluoride gets their bloodstream and and really strengthens the forming teeth.