Identifying a Customized Built-In Element without an is= attribute - custom-element

Question

In my Chessly.github.io project I use Customized Built-In IMG Elements to define SVG chesspieces:
Question: How can I distinguish a regular IMG from a customized IMG?
document.body.append(
document.createElement("IMG", {
is: "white-queen"
});
);
This creates a chesspiece, but does not set the is= attribute
I now explicitly set the is= attribute myself, but since this attribute does nothing and can be set to any value (I use is as an observed attribute in my Custom Element code) it is not a solid way to distinguish IMG elements from Customized IMG elements when walking the DOM.
If I promote a pawn (replace the img src)
<img is=white-pawn/>
with element.setAttribute("is","white-queen")
How can I determine the piece originally was the white pawn?
It still is a white-pawn in the Custom Elements registry.
Am I overlooking something?
Simplified code (with basic SVG shape) in JSFiddle: https://jsfiddle.net/dannye/k0va2j76/
Update: Code (based on correct answer below)
let isExtendedElement = x =>
Object.getPrototypeOf(x).__proto__.constructor.name !== 'HTMLElement';
note! This does NOT catch Autonomous Custom Elements!
maybe better:
let isBaseElement = x =>
Object.getPrototypeOf(Object.getPrototypeOf(x)).__proto__.constructor.name=='Element';

Answer 1

I think adding explicitly the is attribute is the best solution for now.
Else you'll have to deal with the class reference. In your case:
yourElement.constructor === customElements.get( 'circle-image' )
yourElement.constructor === CircleImage //if it's the named class
This supposes that you know the name of the custom elements you want to check.
Else you'll have to go through the protoype chain:
CircleImage --> HTMLImageElement --> HTMLElement --> Element --> Node
If HTMLElement is only the father, it's a built-in element.
If HTMLElement is the grandfather (or grand-grand...), it's probably an extended build-in element.
update
If you use a named class you can also retrieve its name:
elem.constructor.name