The CE mark on a thermostatic radiator valve is a declaration by the manufacturer that the product meets the applicable European legislation. It is not issued by an authority, which is why the supporting documentation matters more than the mark itself.
For thermostatic radiator valves the key reference is EN 215, which defines the dimensions, the temperature setting scale, the control characteristics and the testing methods. A valve claiming EN 215 conformity should be supported by test data from a recognised laboratory.
Materials legislation matters too. Brass parts in contact with drinking water fall under different rules, but for heating valves the relevant concerns are the RoHS restrictions on hazardous substances and, increasingly, the presence of lead in brass alloys.
When buying from a Chinese supplier, ask for the Declaration of Conformity, the test report behind it, and the quality management certificate. An ISO 9001 certificate covers the factory's process control and is a separate document from product conformity.
Taize holds CE, ISO 9001, SGS and DNV certification and can provide declarations and test reports with each shipment, together with material certificates for the brass and steel used in the order.