the problem with engaging with them using tests is they can deny the results. they can say X politician was killed, but was replaced by a body double. A flat-earther, or holocaust-denier, or moon-landing-conspiracist will always have another counter-claim to whatever challenge they accept. There's always an out, due to the number of "variables in the equation", or the number of theories and posts online. One quote I read here (regarding flat-earth) was "If you engage them, you've already lost". :(