Is there any way that answers that seem to have biased sources/references can be marked clearly. This answer to the question did-people-think-the-earth-was-flat, is highly upvoted but I found the sources to be somewhat biased. I left a comment and downvoted the answer, but is there anything more I can do to mark it as biased? If there isn't, can we make a biased voting panel (like the one for close question), where a sufficient quorum of users can declare the question biased?
It is not always easy to decide if a source is biased. Nobody is perfectly objective, if you apply unreasonably strict criteria you can reject any source. I don't care that much about a perceived bias in a source, I care much more if they are correct.
If you think a source is unreliable, commenting and maybe also downvoting is the correct way to go about it. But it is much better to explain in a comment why a source is wrong, than to just assert a bias. If you have a better answer to the question that contradicts the previous one, just post it as a new answer.
Flags are for calling something to the attention of the moderators. Incorrect answers are no reason to flag, that is what downvoting and commenting is for. There is no need for a second mechanism besides voting to judge the quality of answers, and I'm pretty sure the SE team won't implement anything like this.