How does one ensure their knowledge of a field is complete when doing a literature review
This seems like an impossible task.


You're right, it is impossible.  One can really never hope to gain a "complete" knowledge of a field, or even a subfield.  You just have to do the best you can, in the time you have, to identify the most significant basic work related to your area of interest, and work that specifically addresses the questions you are studying.

Part of your advisor's job is to help with this; they should have a broader knowledge of the field, and be able to help direct your literature search and point out any glaring gaps.  But despite your combined best efforts, there will almost certainly end up being significant work that you overlook, or whose relevance you don't recognize.  That is just part of doing research.  Someday you'll learn about it and wish you had known it earlier, but all you can do is move on from there.