Watch for the long vowels. Pronouncing "about" as "aboot" is a dead giveaway for most Canadians I've met.
I'm not able to hear the difference - you all have long vowels compared to me! Probably the same way @Bullgrit can't hear the difference between English and Welsh. We're simply not used to the accents enough to hear the fine-tuning which seems obvious to locals.
Outside of Val-speak (like, fer sure, no way!) I'm not familiar with a California accent, and I'm a native (southern) Californian -- curious what people identify as a California accent.
I don't think I could pick one out. It would just be "generic strong American accent" to me. Really, it's just the Southern drawl and the Noo Yoik accents that I can pick out; the rest all sounds the same. Just like you probably can't distinguish between a Yorkshire and a West Country accent, or a Geordie and a Cockney accent.
In English English (British English? UK English? Who owns English?) it's the mixed common terms that get me. The pants-trousers divide is particularly humorous; chips-crisps has caused me frustration; I never know what I'm getting if offered a biscuit; and corn-maize has me downright bumfoozled.
Don't worry; it works both ways!
