First of all, you can check Signature Panel in Adobe Reader [1]. It allows you to check the validity and integrity of an electronic signature and see all of the details about the certificate, the Trusted Service Provider, who has signed the document, etc.
1. Adobe Signature Panel
Also, you can use Digital Signature Service (DSS) Demonstration WebApp [2]. There you can also get a report regarding your signature.
2. EU DSS Demonstration WebApp
The third option is to use qualified trust service providers that specialize in validation services. QTSP status granted to certain providers guarantees that the provider follows eIDAS requirements; therefore, the user of the service benefits from higher legal certainty. Check the EU Trusted List to find a provider.
Find more information about validation here.