You need to find out the country of the journal (not the publisher). For example, journals issued in the USA (published by Elsevier, for example) will use American English. However, journals issued in the UK or Australia (also published by Elsevier), will use British or Australian English.
Consistency is what is important. You can stick on to any one but be consistent throughout the manuscript. I remember reading this in the instructions to author part of a reputed elsevier journal.
You need to find out the country of the journal (not the publisher). For example, journals issued in the USA (published by Elsevier, for example) will use American English. However, journals issued in the UK or Australia (also published by Elsevier), will use British or Australian English.
Good answers so far. Srinivasan importantly stresses consistency. Always look to the author instructions for any given journal on style etc. If in doubt - look at a recent published article - and 'copy' the noted house-style. Samy makes the point that US-based journals will use American English - and that is certainly mostly the case - but not always. It tends to get even more blurry with books and book chapters - especially edited tetxs. They may have multiple international authors using their 'own' style and a common format has often to be agreed - which may be a mix or an adaptation - espeially with regard to say citation style.
In journals without copy editors, in publications of international organizations, in conference proceedings, etc., rather often both American and British spellings are used in different contributions or even in one and the same paper. Many people do not care about this. But my impression is that American English is used more often than British.
Wolfgang R. Dick - I'm afraid I have to disagree with your point that "many people do not care about the mix of British/Australian and American spelling" in the same manuscript. Editors care a lot. Yes, this could happen in poorly edited articles that are not published in prestigious journals, but we never ignore this. This area is critical in publications issued in Australia, the United Kingdom and the USA. We never ignore consistency in spelling.