What's a word for an instance in which one has an opinion about something without having tried it?

Are you thinking of "prejudiced"?

You might go with "closed-minded" or "small-minded" for the sense of not being interested in learning anything that might change their opinion, or perhaps "willfully ignorant" if you believe they've made a conscious decision not to educate themselves further about the matter.

I would describe what such a person is doing in that case as "speaking from ignorance" (in opposition to speaking from experience), but I can't come up with an existing word or phrase to describe someone as a person who routinely speaks from ignorance. To coin a phrase for it, I might go with "an oral flatulator."

Edit:

The speaker has a preconceived bias.