Validation of electronic signatures in Swiss jurisdiction (according to Swiss Signature Act ZertES)
You can use Validator.ch, which is a service of the Swiss Federal Administration.
Validation of electronic signatures in EU/EEA jurisdiction (according to EU regulation eIDAS)
Option 1: You can use the signature verification of the Austrian Regulatory Authority for Broadcasting and Telecommunications (RTR).
Option 2: You can use Validate a signature by the European Commission.
Important Disclaimer: So that you know, EU member states' validators still need to be harmonized. Specific validation portals can validate qualified electronic signatures as invalid even though they meet the requirements according to EU regulation eIDAS. The EU is responsible for working on harmonization.
Note that you can only validate qualified electronic signatures (QES). There are no validators for advanced electronic signatures (AES).