I usually check who set the property on Telerik controls by using Snoop.
Pin the element with Control + Mouse hover. Then select exact element in visual tree, select the suspecting Style property and use Delve button to see properties of Style property, namely look for x:Key.
When you find the key (usually RadSometingStyle or Template), open the install directory of Telerik controls, browse to Themes foleder and search over the all xaml files there. You'll find the style definition there. Good luck... :)