Though mastering different accents is not a necessary undertaking on the path to becoming a successful voice actor (or any other kind of actor) it is still a cool and fun skill to have. Being able to convincingly and consistently perform another accent will open doors for you and widen the range of opportunities available to you.
Naturally when trying to learn an accent, you must know what that accent truly is and to be able to listen back to yourself objectively to know whether you are doing it justice. There are abilities you'll want to master along the way, such as being able to perform the accent at different volumes and with different emotions. But there is one way I feel that you can know for sure that you can confidently say that you can perform an accent. Ready to hear it?
I believe that a person can comfortably perform an accent when they can comfortably read aloud in that accent – without really thinking about it.
The mind is an incredible thing. Any time you are having a conversation with someone, your brain is simultaneously making your mouth utter the words you are currently speaking whilst also composing the words you are about to speak. When we are spontaneously performing an accent, our brain (very understandably) has a habit of choosing words to come out of our mouth that it knows how to speak and picks combinations of words that, in that accent, will flow from our mouths smoothly.
This is precisely why reading aloud in another accent is such a great thing to do. It gives you a chance to read somebody else's words, for you to be speaking in an accent but not be prepared for what is next. It allows you to see what you can do with it and shows you where the hurdles are – the ones that your brain will usually steer you well clear of. Unless you're lucky and multi-talented, as an actor you almost always are performing somebody else's words.
I bet that with certain accents you find yourself saying the same words over and over again, safe ones that accentuate certain sounds distinctive to that accent. Maybe you repeat stereotypical catchphrases often and have gotten very good at saying the word "potatoes" in an Irish accent as an example.
Now, I understand that for some people, there are accents that they have been able to perform for their whole lives. Maybe their parents came from another country or one of their favourite TV shows growing up was from abroad. Nevertheless, I strongly recommend taking this test regardless of how you came to do the accent. I personally found that some accents I had been performing my wholelife, very comfortably, completely fell to pieces once I tried reading aloud with them. It was a real wake up call. I also think this is why sometimes you'll hear bad accents on TV shows and movies. The person can do it comfortably when they're winging it by themselves or even with a few lines… but once they have a lot of somebody else's words, the illusion is shattered.
So read a random article on the Internet or from a magazine in a different accent. If you find that suddenly your performance of that accent isn't as fluid, that you're more self-conscious or it isn't sounding as good to you anymore, don't panic and just understand that it needs a bit more work. That's all. If you are performing it comfortably and it's sounding believable, then it means you understand the accent truly and on a deep level, it means performing the accent is no longer a conscious process and this frees up your thinking so that you can focus on acting.
Why not test out one of your accents by reading this article a second time?
P.S. Reading aloud is a great exercise that improves eye-brain-mouth coordination and has many other benefits. I highly recommend read (aloud or to yourself) Yuri Lowenthal and Tara Platt's incredibly awesome book Voiceover Voice Actor to see them all and learn tons of other VO stuff.