Determining common knowledge in an unfamiliar field
By far the simplest way to get around this problem is to simply give it to a colleague who is more well-versed in the field and have them judge which statements need to be backed up by citations and which don't.

The next simplest way is to read papers on a very similar topic to yours (which, no doubt, do exist) and see what they cite.

On a related note, my advisor gave me the advice of "when in doubt, cite." His point was that the only real downside of an unnecessary reference is added length to the paper. If you get to that point, and the only thing left to cut is citations, then you can start to worry.