· Three-pedal evangelist
2020 Premium 6MT
Yes, dry rot is visible. It starts as tiny "micro" cracks and they get larger over time.Can you even tell just by a visual exam? I wasn't able to find a good source but I recall that any rubber tire over seven years from manufacture could be dangerous.
I doubt there is any blanket time that a tire can be considered unsafe. Dry rot can happen in as little as 4 years or may not happen for much, much longer. It can also depend on the composition of the rubber compound. Temperature and humidity swings accelerate it, so tires on a vehicle that is garage kept will last a lot longer before getting dry rot.