It is typically easy to spot someone who does not have the facts on their side or is not solid in their convictions. It really has nothing to do with a person's intellect or even their political leaning or what they personally believe in. It is the way that they argue.
One of the first things that someone will do that does not have facts or a strong conviction is to repeat, if not verbatim, than pretty close, information they have seen on tv, heard on the radio, or read on the internet. It is pretty easy to spot the people that do this, because, when pressed they will deny that the verbiage they have used came from anywhere but themselves. There is no harm in repeating information you have heard or seen or read, as long as you give credit where it is due.
The other sign that the person you are are debating is actually only using their opinions rather than facts is just how quickly they get defensive about their point. Now, this should not be confused with defending one's point. The person that gets defensive does not answer back with facts to back up their point, they get personal. They will often throw insults, call names, question the other person's intelligence; but they will rarely if ever actually respond to the person that has challenged their information with something actually dealing with the original point. They will often claim that you are feeling something you are not and then try to exploit that to their gain.
They will also demand that someone disprove their point. They, for whatever reason, feel they do not have to ever prove their point, just the fact that they made the point makes it true.
When all else fails they will change tactics up completely and try to change the focus on what they were arguing to begin with. They accomplish this a couple of ways: they deny that they ever made the point to begin with, they will change their original point just enough to change the original point but also give them an edge in the debate, and finally they will completely change the topic and try to push the discussion to something else entirely.
They will also change your argument or just completely make up an argument for you and then attack you over it and make claims that what you are saying is not even true (never mind the fact that you never made the comment to begin with).
Last but not least when a person is faced with the idea that their opinion or belief is actually wrong they will often ignore that they have been proven wrong and continue to push their point.