From the screenshot, it looks like the tooltip popup is likely with respect to the "slider" WebElement that is passed into the JS executor in the try block, not the highlighted line at the bottom regarding '.ui-slider-' which is obvious ByCssSelector or ByClassName. And that tooltip shows found by XPath.
And from what I can tell from the tooltip, it looks like the found by method is available in the object/class member "foundBy". However, based on my experience with Eclipse, which is what the screenshot appears to be for, foundBy is denoted as a red square, which I believe means is a private member, so it's likely not accessible to users. If it were though, then I would think one could extract it via slider.foundBy, although one might need further processing of the data as foundBy appears to be some object type itself, not a simple string.
And if it is private, perhaps one could modify/extend the applicable Selenium classes (and recompile as needed), to change foundBy to be public or protected.