To quickly search the current project for references to the currently selected symbol, use the Lookup References command (Ctrl+/). See: Lookup References. For example, click inside an identifier such as "BeginPaint", then invoke the Lookup References command. Source Insight will open a Search Results window, which lists all the places you used BeginPaint in your project. Each matching line listed in the Search Results window will also have a source link to the location of each line containing the word you looked up.
The Lookup References command is context sensitive, so that if finds only the correctly matching references, given the scoping context.
The Lookup References command and the Relation Window use a special search index for speed.
The Relation Window also shows references to the symbol at the cursor position automatically. You don’t need to use the Lookup References command unless you want to put the results into the Search Results window. See: Relation Window.
You can have Source Insight automatically highlight references to the symbol at the cursor position within the source code window. This is controlled by on a per-file-type basis using an option in Options > File Type Options. See: File Type Options.