In a video how do you know that the iron rod is a negative charge or positive charge if there - Physics -

Question

In a video how do you know that the iron rod is a negative
charge or positive charge if there is any type of object so why do
we not use that object instead of electroscope

nitish.gupta · Accepted Answer

Since iron is an electronegative element which has the tendency
to gain electrons, so that is precisely why a rod of iron is termed
as a negatively charged body We decide the nature of polarity of an
object by seeing its electronic composition and the valency

In a video how do you know that the iron rod is a negative charge or positive charge if there is any type of object so why do we not use that object instead of electroscope