Loop through all registered widgets and find those that are not installed from assembly with name Telerik.Sitefinity.* . To check this in the UI go to Administration->Settings->Advanced->Toolboxes->Toolboxes->PageControls->Sections.
To query those refer to the API for working with this configuration section. Sample here
var config = ConfigManager.GetManager().GetSection<ToolboxesConfig>();
var pageControlsSection = config.Toolboxes[
var allSections = pageControlsSection.Sections;
var builtInWidgets = section.Tools.Where<ToolboxItem>(t => t.ControlType.StartsWith(
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and vote to affect the priority of the items