For those who are worried on their wheels or tyres limit, you can refer this diagram of cross sectional cut out or how the tyres made out of many layers.
The most important part is the steel breaded wire layer. If you can see any exposed threaded or steel braided layer part of the tyres, it means that it has reached its maximum worn.
By right, you must replace the tires before you can see the inner layer exposed.
Beside that, if you see the tire bulging at any area especially the sidewall, it indicates the inner structure has weaken and you must replace it immediately.
Any torn, damage, scratch that does not exposed inner layer and not bulging is consider within limit. But it is recommended to replace the tyre earlier when you see the thread part is getting too thin which will affect the tyre performance during raining.
TYRE WORN LIMITS INSPECTION DEFECT CLASSIFICATION