You can do two things, with specific drawbacks
1. There are the methods getComputedStyle/getPropertyValue. Together they will tell you the current value of a specific CSS property.
BUT: It will always give you a result. This is because all of the CSS properties will have values even if they have not been set explicitly. So you can't tell the difference between border-radius 0 that has been set deliberately and border-radius 0 that has been set by default
2. Each element will expose its style attribute/property. So if the border-radius has been set using style="border-radius: ...." in HTML, this will give you the value
var myValue = element.style.borderRadius;
BUT: It will only work for inline HTML style attributes