I've been looking for any standard to evaluate radars capability in electronic warfare. In other words if you buy a radar is there any standard to test and evaluate its ECCM capability?
Due to the sensitive nature of the topic, one would struggle to find an open standard to assess against. However, there are many figures of merit which can be used for comparative analysis between systems. The ECCM improvement factor (EIF) proposed by Johnston would be a good starting point. A discussion between Johnston and Li regarding their respective work in the area, with many useful references, can be found at https://digital-library.theiet.org/content/journals/10.1049/ip-f-1.1985.0044
The shortest answer is: No, there are no standards. ECCM is a very broad topic and depends heavily on small details of the actual radar under investigation.
There are, however, several good text books published on the subject.
Some doctrinal aspects of ECCM are standardized but such documents described only general aspects. As an example, UNO International Ammunition Technical Guidelines (IATG) include few requirements for safety distance from ammunition storage to radars in the context of ECCM.