To assess the wear of a tyre, it is necessary to inspect it carefully from all sides. The key is to check the depth of the tread - that is, the top layer of rubber in contact with the road, which by law must be no less than 1.6mm.
This level is indicated by the TWI (Treadwear Indicator) markings inside the grooves running through the tread. When the rubber is at this level, the tyres should be replaced immediately.
To be on the safe side, replace your tyres when the tread depth exceeds 3mm for summer tyres and 4mm for winter tyres. When assessing the level of wear on your tyres, also check for irregularities or damage such as bulges/bumps, dents, abrasions, cuts or signs of evaporating rubber. If you notice any of these symptoms, consult a professional.