I run into this a lot.
I probably can’t tell you the specific region of someone’s accent, but what I can tell, is when you’re chewing the words or when you’re talking around the words.
If the role requires an accent, make sure you’re comfortable with it. It’s a good idea to have an arsenal of accents in your toolbox, but what’s more important than the diphthongs and diction, is your comfort level with the accent. You could be hitting every vowel perfectly, but it will still sound awkward if it’s not comfortable.
And if you’re focusing on the accent, and trying to get every word right, then you’re not paying attention to the scene, and neither am I. It’s like watching someone walk a tightrope over the Grand Canyon – we’re both waiting with bated breath, watching each toe grab the wire – instead of looking at the Grand Canyon. I’m just watching to see if you can get through the scene, instead of watching to see how you make the scene special.
It doesn’t hurt to ask casting if you can do the scene in your natural accent. We know it takes a little time to get comfortable with the words – it’s even easier to say yes to that request if you have tape or footage of you doing the requested accent that you can send to casting, proving your skills in that area.
On the off-chance the accent is integral to the character – you gotta let go of trying to get it perfect. At least for the audition. Do it to the best of your abilities, and focus more on the scene – if they like your acting, and your choices, they’ll hire you a dialect coach. And if they don’t have the time or money to do so – then you gotta let go of this one. They need someone who already had that accent in their wheelhouse – and you’re not that person. It’s okay. There will be other roles.
More often than not – my team will let go of an accent before they’ll let go of an actor. The choices you make, the vulnerabilities you hide, the insights you share – if those align with the character, that takes priority over a tone or a twang.
Also – on a personal note – your job as an actor is to portray real people on screen. Are you not a real person? Is your natural accent not real? If you’ve got a particular accent, chances are other people will too. Represent them.
On another personal note, if you’ve been hired to play someone with an accent, you’re representing them, you’re stepping into their shoes. I’d suggest you work on it. We all know the internet will have a critique.