To verify if SSL key match with the private Key, run following commands and compare resulting hash, it should match if SSL and Keys are matching.
Verify Private Key
openssl rsa -modulus -noout -in MY_DOMAIN.key | sha256sum
Verify SSL
openssl x509 -modulus -noout -in MY_DOMAIN.crt | sha256sum
Verify CSR
openssl req -modulus -noout -in MY_DOMAIN.csr | sha256sum
PHP Script to verify private key matches SSL certificate?
ssl