You can't ask if the culture has been changed by the user but you can request a culture that has not been changed by the user.
new CultureInfo(CultureInfo.CurrentCulture.Name, false);
or
CultureInfo::GetCultureInfo(CultureInfo.CurrentCulture.Name);
In either of these, the UseUserOverrides property will be false.
Only the default culture can have its data overridden by the user. An administrator can override the data in any culture by installing a custom replacement culture. You can detect that by checking the CultureTypes property for the presence of the CultureTypes.ReplacementCultures flag. There is no way to get the original data in that case.
Note that if you are actually trying to write code that changes its behavior based on this, you probably are writing code that will be broken by future releases. The data does change between releases as bugs are fixed.