When Glass Rod and Silk rubbed, Charges aquire as follows: Glass +ve & Silk -ve
When Glass Rod and Fur rubbed, Charges aquire as follows: Glass -ve & Fur +ve
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Like repels like and opposites attract. So if glass in one experiment gains a negative charge but in another, gains a positive charge, there will now be an attractive force between them and you can conclude that they have opposite charges.
How do you decide which is positive and which is negative?
Easy, be Ben Franklin in the 1800s and when rubbing glass with a silk rod, decide that a positive charge was moving *into* the glass rod and therefore, the glass is now positively charged and the silk cloth negatively charged.
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