I wanted to check that the private key (not the mnemonic) I had written down correspond to the opt-in address of my current NEM account. I did a small script published at https://github.com/rb2nem/symbol-optin-validation doing just that.
Please be paranoid, use it at your own risk. If you spot a bug, please let me know.
You can run it with docker using an image from dockerhub:
docker run --rm -it --network none rb2nem/symbol-optin-validation
This runs a container without network access (to limit risks of leaking your private key) and it will prompt for a Symbol address, then for a private key (without printing it), and will print if both correspond or not.
Hope this can help