Answer
See the detailed answer below.
Work Step by Step
We know that the like charges repel and the unlike charges attract.
So any two objects that were charged by a plastic charge will repel each other. And so do the two objects that were charged by a glass charge.
So to test the new type X-charge, we need to approach it to a freely moving object that is charged by a plastic charge and then approach it to another object that is charged by a glass charge.
Let's assume that the charge X is on a rod.
So we need two extra rods that are suspended from the middle by a rope one of them is charged by a glass charge and the other by a plastic charge. The two rods are away from each other enough distance that they are not affecting each other.
We need to approach, with no touching, the X-charged rod to the glass-charged-suspended rod.
They must be attracted since both are carrying different charge types.
The same thing must happen for the plastic-charged-suspended rod, it must be pulled toward the X-charged rod.
But both will be attracted to the X-charged rod if the rod was uncharged as well.
So we have to approach the same X-charged-rod to a neutral-suspended-rod. If the natural one is attracted to the X-rod, then the $X$-rod is charged by an X-charge.
And if the X-rod is not charged, the suspended rod will remain stationary with no movement at all.
