First, make sure the document only includes text to be proofread. Make a copy of your file and delete all text that does not require proofreading, such as references. Please see the below FAQ entry for what to exclude.
Using the trimmed document to be proofread, check the word count. In Microsoft Word, the simplest is to look in the bottom left for the word count:
Alternatively, click on Word Count under the Review pane:
Please note that if you have text selected, both of these methods will only count the words in the selection.