After studying South-East Asian men for some years, I find it quite easy to distinguish the various nationalities. It defies easy description, but they look different, and often dress different. A good way is to have a look at their mobile phone screen. Recently on public transport, I sat next to a cute boy, who looked very Myanmese. And looking at his mobile phone screen, I found this confirmed by seeing Burmese script. Listen to them speaking is another way.
The only I cannot distinguish is some Lao boys from Vietnamese. It doesn't come as a surprise that some Lao look Vietnamese. Or the Vietnamese are aware of their bad reputation and try to pass themselves off as Lao? Next time I have one of the, I will ask in detail where they come from, or even have some Lao text on my phone to see if they can read it.
When talking to them in Thai, I cannot hear if they speak with an accent. But as I have traveled all over Thailand, I ask for their home province. While the one or other Myanmese boy will say "Chiang Mai" and the one or other Cambodian boy will say "Surin", when I ask for their home district, they falter.