The only way I know of to do this would be to call the ValidateString of the HttpRequest class. Unfortunately, it is private, so you have to do it with reflection.
I have not tested this - it is not exactly the same code I use. it is adapted from code I wrote a few years ago, so maybe Microsoft has something new which make this easier.
MethodInfo _validateMethodInfo = typeof(HttpRequest).GetMethod("ValidateString", BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.InvokeMethod);
foreach (string key in Request.Form.AllKeys) {
if (key = "< skipped field name >") {
continue;
}
object[] parameters = { Request.Form[key], key, "Request.Form" };
_validateMethodInfo.Invoke(Request, parameters);
}