If there are no gauges fitted there are some other accurate ways to tell brake temp visually:
Remeber the fish tank colour strips/stickers that changed to show the colour on the side? The were accurate to 1*c so something similar could be made or look at
Rally/Race car or high performance brake kits from Brembo or AP racing have 3 colours green, yellow, red paint that goes black when the max safe temp was exceeded and therefore the disks have to be changed.
This is especially true for carbon/ceramic applications as they have very high oxidation temps that when exceeded tend to burst into flames and braking effort fades gracefully away.
Look at the problems Porsche is having with its Ceramic Composite Braking system.