Before you start your review keep in mind the purpose of an index:
An index is a key to significant treatments of subjects, requiring harmonization of vocabulary and collocation of treatments described differently, especially in multi-author works. It also has to supplement the author’s terminology to accommodate the language in which the reader may formulate questions. (from “Last But Not Least”, a leaflet published by the Society of Indexers)
As an indexer is only human, checking for typos and other anomalies is the first step you should take.
Are the major ideas and concepts discussed in your book easy to find in the index? Just scan through the main headings and check if the key concepts are present.
3. BALANCE of COVERAGE
Is each part of the text given its appropriate treatment in the index? Arbitrarily check a few concepts at the beginning, middle and end of the text and see if they are evenly indexed.
4. CONSISTENCY of FORM
Are main headings and subheadings consistent of form? Is the same wording used for the same topic in all of the headings? When an indexer has opted for communication instead of communicating is this done consistently. Is the structure of subheadings, whenever possible, repeated under different main headings?
Randomly look things up in the index, and see if you can find it on the pages to which you were sent. Does it work the other way around? Locate some important topics on a few pages and see if you can find them in the index.
If at this point you searched the pdf file for a certain word and the index doesn’t seem to cover them all, remember that passing mentions are not indexed. An index is not a concordance of all the words in the text, but a guide to substantive discussions in the text.
6. MAIN HEADINGS & SUBHEADINGS
Main headings shouldn’t have long strings of page numbers or very long page-spans. Subheadings should break down the main heading into manageable portions. Are the subheadings logical and is the wording succinct but unambiguous?
Are there enough ‘see’ cross-references? The point of an index is to provide multiple entry points into a text, so are there several acceptable terms (synonyms, for the same concept? If readers are looking under cars, it’s no help to them to have entries at automobiles. A ‘see’ reference is there to point them from cars to automobiles.
Are there enough ‘see also’ cross-references? The main purpose of ‘see also’ references is to lead users to closely related additional topics in the index. At least the main connections between topics should be indicated.
Check if cross-references work as intended and lead to headings that exist.
8. INTRODUCTORY NOTE
Does the index start with an introductory note to give guidelines for use of the index(es)? For example, that illustrations are indicated with italic page numbers (225), or that tables are referred to with a ‘t’ behind the page indicator (225t). If there’s more than one index, the introductory note should state this clearly.