I know of no way that cannot be defeated.

You can check the document properties.
File > Properties > Advanced Properties
Look under the summary and statistics tabs.

Look at the Author, the Company, the template, editing time and the creation date. If two documents have the same creation date, to the second, they are likely copies. The creation template probably will be normal.dotm for both.
Note that the same creation date and author tell you that the document started from one base, but does not tell you at what time or how much they diverge. A modified copy will likely have a substantially greater editing time. (A greater editing time may just tell you, though, that more time was spent on the document.)

.doc files or .docx files?
.doc files can be looked at using a code editor to look at the binary structure
This can give you names of people who edited the document.

Search on the Internet for Microsoft Word metadata.