While the variant spelling feature is generally quite effective, it is always possible that your term will have variants in the database that it does not return. A few rules of thumb might be useful if you're trying to guess at a variant:
Click on "Search" on the EEBO opening page to enter the Basic Search form. In Basic Search, there are several search fields you can use to begin your search in EEBO:
Include: This box on the Advanced Search page contains check boxes for "Variant Spellings" and "Variant Forms." Which ones should you use?
Variant spellings are quite common in Early English books, so select this box to receive the fullest results. For example, searching "dog" with the Variant Spelling box checked will return documents containing "dog," "dogg" and "dogge." But do not select the box if you are searching for one specific spelling, or using advanced truncation and wildcard operators.
The Variant Forms option is the broadest search that will search for different inflections of a word (for example, variant forms of "murder" include "murdering" and "murdered"). If you select "Variant Forms," an EEBO search will also automatically include variant spellings.
Keywords: The Keyword(s) field will search the full text of documents as well as the citation information for your search term. If you want to know all of the possible variant spellings for your keyword, as indexed by EEBO, type in your term and hit "Check for variants." Then select the checkboxes by all of the variant spellings you want it to search (or just select the Variant Spellings box at the top of the Basic Search page).
Limit To: The Limit to drop-down menu allows you to limit your search to documents with images (most of the collection) or documents with keyed full-text (a small portion of the collection).
Author Keywords and Title Keywords: The Author and Title search fields will search for keywords for the author name or title. To find the exact indexed name or title, click on the "Select from a list" link next to each keyword field.
Subject Keywords: This will search for terms that may be subject terms attached to the documents you want. But for the most effective search by subject, type in your subject term, and then click on the "Select from a List" link. This will show the list of indexed subject terms in EEBO that most closely match your term, and select the checkbox(es) next to the term(s) you want to use.
Bibliographic Number: The Bibliographic Number field is used if you have a citation for a document from the Short Title Catalogue.
Limit By Date: Limit by Date allows you to limit your results list by date of publication.