Let’s use the example of a watch.
Suppose for whatever reason you were not aware of the existence of other humans or factories or any methods known to create watches. Perhaps this could be the result of you being abandoned by your parents or experiencing memory loss or some other weird reason that resulted in this predicament. The reason doesn’t matter for the purpose of this question. What matters is you do not know that other agents like you exist, do not know what factories are, and have never come across anything similar to a watch before.
Now, suppose you come across a watch. Are you now justified in thinking that it was designed by a human, even if it is as a matter of fact designed?
I think something that would make this question even more interesting is if one is not human or one is some kind of robot or even some mind-entity floating in space who has no sort of interaction to the object except for sight and/or only sensory functions. What then?