Caveat: I'm not a trained Historian, just someone who's read a lot of history books over the years, and has learned this the hard way.
First off, every writer has bias. Know that going in.
So if you want to find your writer's bias, you have to learn a bit about them. Where did they grow up and go to school? Are they from an ethnic minority in their country, or from the dominant group? Did they grow up benefiting from the power structure, or oppressed by it? Are they active politically (and thus liable to be pushing an agenda for their own benefit)?
Next you have to learn a bit about the country (major chicken-and-egg problem here!). If you know a bit about what's going on politically in a country, you know what people there want to believe, and what is uncomfortable for them.
Armed with all this, there's one good rule of thumb: there's roughly an inverse relationship between how uncomfortable information is for the writer and his presumed audience, and how much you can trust it. I'm not saying you can't trust any positive statements a person makes about their own culture. But if its exclusively positive, and/or exclusively negative about some culture that isn't their own, your mental shields should go up.
To take an extreme example from perhaps your own history: I'm not an expert on SE Asian history. But if I see a Khmer writer (particularly an avowed Communist or Socialist) who insists the Killing Fields have been way overblown, I'm going to be more than a little bit skeptical. Its likely to be both in his own interests, and something he'd personally prefer to have been true (regardless of the facts).
On the other hand, if the same thing is said by an expatriate historian of ethnic Chinese Cambodian or devout Cambodian Buddhist descent, and his work isn't chiefly known due to promotion from folks in the group in my previous paragraph, I'd be a bit less guarded.
(Pro tip: this works for yourself too. Be extra careful about stuff you want to believe)
