I think usually the best way is to ask them what kind of betting odds they would take. Although in this case the difficulty in verifying might be a problem. But maybe if you argued that a 100% certainty means they should be willing to lay infinite odds that might get them thinking.
I think usually the best way is to ask them what kind of betting odds they would take. Although in this case the difficulty in verifying might be a problem. But maybe if you argued that a 100% certainty means they should be willing to lay infinite odds that might get them thinking.