Your verdict should be based on the evidence presented and your opinion of that*. Your personal opinion can be different.
This is really quite simple.
I can look at someone and comment that they look like an alcoholic, but if it was in court in regards to say drunk driving for that to be "beyond a reasonable doubt" the prosecution would need to present evidence that the person was in fact drunk in the legal terms at the time of the alleged offence, not just that he liked a drink on a regular basis and was found drunk later that day.
You can go to your GP and he might say "well it looks like that lump is nothing to worry about, but I'll send you for a test", that is basically the same thing, the GP's initial opinion is that it's unlikely to be anything serious, but because he's a professional he's going to make sure the evidence backs that up.
It's also why it can be quite hard to get a straight answer from a scientist, as they might say "well I think it's possible, but I don't know" or "I can't rule that out, but I think it's unlikely" if you ask them a question about something where the evidence is unclear.
*To make it even more fun, depending on if it's criminal or civil the burden of proof varies, so "beyond reasonable doubt" is basically there is little/no doubt about what the evidence shows, "on the balance of probabilities" the evidence shows it's a better than 50% chance, or personal opinion which can be "that guy's eyes are too close together" or "he looks like the guy that mugged me 20 years ago".