Ask for the reason
"How do you know that?"
Recorded by Plato · Laches 190c; Apology 21c–d
A confident tone isn't a reason. Ask what the claim rests on: a study, an experience, a hunch, someone's say-so. Then you know what you're actually weighing.
'And of that which we know, I presume, we can also say what it is.' (Laches 190c). In the Apology he describes questioning a man reputed wise who 'seemed… to be wise to many other people and especially to himself, but not to be so' (21c).