How do I know whether my private key has been stolen?
If your private key was stolen, then the thief can provide you with definitive proof that he possesses the key---for example, by spending your funds.
But unless you have such proof, all you can do is guess whether your private key has been stolen. There's no definitive way to prove that your key hasn't been stolen.
Usually, if you have enough evidence to worry that your key has been stolen, it's a good idea to take the steps you would take if you knew it was stolen. For example, generate a new safe private key and address, and use your old private key to send funds to the new safe address. That way your money is safe whether or not your old private key was stolen.