Check this:
https://github.com/ReinProject/bitcoin-signature-tool

Once you download and extract it on a PC, it can be run offline. If you put your private key in there, it will generate the public address. It is mainly useful for signing a message from private key, but you can use it to validate the key as well.

If you're not familiar with how to use it offline, you can check this tutorial:
https://steemit.com/bitcoin/@bitsignal/how-to-sign-a-bitcoin-message-using-your-private-key