What makes you ask that of them? Are you already approaching their point from the opposite angle and want to find out if they've got a bias rather than challenge their point directly?

Are people not allowed to criticise a publication any more without throwing in a token caveat of: "by the way, publications on the other end of the political spectrum do this too, and I equally don't approve of that" to ground their perceived neutrality before someone challenges their argument rather than their motivations?