Website: https://eccver.github.io
This website uses Javascript with Elliptic library (https://github.com/indutny/elliptic) in order to verify ECDSA Secp256k1 signatures.
If you don´t trust this website for any reason you can use this similar one: https://kjur.github.io/jsrsasign/sample/sample-ecdsa.html or OpenSSL: https://stackoverflow.com/questions/22856059/openssl-ecdsa-sign-and-verify-file