To test the tweeter, first, without plugging in an instrument, connect your amplifier to your speaker cabinet as you would normally, then turn the amplifier's power switch to the "On" position. Rotate the treble and master volume controls to maximum, at which point a loud hiss from the tweeter should be audible. Rotate the tweeter attenuator control back and forth and listen for increase (clockwise) and decrease (counter-clockwise) in volume of the hiss.
If you don't hear any change as you sweep the attenuator control, you may have either a blown tweeter diaphragm or a fried attenuator "L-Pad." A blown tweeter diaphragm is more likely, so you should order either a replacement diaphragm for the horn or a replacement piezo tweeter depending on the model of SWR speaker enclosure that you own.
FMIC no longer carries replacement parts for SWR products, however your local amplifier service center may be able to find a resource for comparable replacement parts.