Yes they are internal to my application, that doesn't prevent me from wanting to write a safe-code onto what format we expect the content of a property file.
But if you don't know how to do it, no one will be mad at you, just avoid answering for nothing please.
I'm answering to help you with good software design.
Code to check that your application is correct should be in unit tests and integration tests, not in the business logic. Do you also write code to check that return values from method calls are correct? Methods guarantee correct results if the input is correct. Resource bundles should guarantee that their contents are correct. They are part of your application, not part of the application input.
Writing code like this leads to bloated, poorly maintainable applications.
Take my advice or leave it, I won't be mad at you.
I think it will be appropriate to check the values in your application code. At the place where you are retrieving value for a particular key, we can add a check that if the value contains "_" then throw Invalid property exception. Else there is option of extending ResourceBundle class and override handleGetObject() and getKeys() method to provide your own implementation, but I still prefer first option as this will be easy to handle and debug.
You can just read all entries from the resource bundle using handleKeySet() and getString(), and using String.contains() to check for illegal characters. Just make sure there aren't actually labels that are supposed to contain these characters.
Resource Bundles exist to externalise the message Strings from your source code to enable you to change them without having to create a new build of your application. If you have a programatic runtime dependency on the value of those message properties, then that String could just as well be hard coded into your application. If you have business logic written around the value of those message properties, then you're misusing them. Resource Bundles should contain Strings for presentation on the UI only. Otherwise what you have is a configuration value, which should be stored in configuration files not resource bundles.