To know which documents your queries are going to return means that you need to know exactly how your query is being parsed by Lucene.Net. Full text analysis is a great baked in feature of RavenDB, but I have found that they Lucene.NET standard analyzer that parses full text fields can sometimes return surprising results.
This Lucene.Net 3.0 Analyzer Viewer is an update of Andrew Smith's original version for Lucene.Net 2.0. This update now allows you to view the results of text analysis for the same version of Lucene that RavenDB is using. This simple tool can be invaluable to debugging full text searches in RavenDB!
This tool also comes with my Alphanumeric Analyzer built in.