A common problem many people ask is - should you test private code? In short, you shouldn't. You should always test the public api of your code where possible. This is not always easy. Based on the context of the code in question there are a few options available.
Either don't test the private code or rely on manual testing. This will not be ideal in many cases, but if the code is covered in higher level tests you may be able to get away with it. If the code will be stable, short lived or low risk you can default to this option.
Test via Public Tests
Simply test the private code by adding assertions or verifications to exisiting public behaviour tests. If the setup requires a lot of work, many edge cases or much duplication you may want to avoid this technique.
Make the Code Public
Once public, the code is easily testable. Are we making this code public just for the sake of an automated test? Yes, but there are valid times to do this. Providing the behaviour is logically part of the object in question there is no harm, the single responsibility principle is not violated.
Interfaces can be used to control visibility here. For testing you always use a concrete instance, while your production code should hold references to interfaces only. To simply hide the method, don't add it to the interface. For dynamic langauges this is as simple as "don't invoke it" or relying on naming conventions to denoate implementation details.
Make a Public Class
When single responsibility principle would be violated in the technique above, this is your other option. Beware the power of just adding a new class and making it public. While it will allow testing in one place, each public dependency you introduce further increases coupling.
If the code that needs testing is a service, the act to introduce a public object should be considered thoughtfully. Once the class is pubic, you simply need to verify the use of the class, rather than what it does. However Value Objects can help limit the tests you need to write entirely and should be used whenever possible.