It didn't occur to me that 'belief' might have weird connotations since I usually mean it in the purely epistemic sense; a different phrasing might be 'putative knowledge' which sounds much less nice to me. It seems someone dubious about your assertion's value might even have to call it 'putative information' rather than 'information', so I'm not sure that helps much.

One of my mentors once suggested "So, what led you to that conclusion?" as a relatively neutral way to probe the origin of a belief, without connoting disbelief or disagreement.