You can use the FindReferencedEntities navigator for that.
var navigator = new FindReferencedEntities(delegate (AstNode node, IEntity entity) { ... });
var resolver = new CSharpAstResolver(parsedFile.GetResolver(compilation, method.StartLocation), method, parsedFile);
resolver.ApplyNavigator(navigator);
'method' is the MethodDeclaration you want to look in.