Have you tried using the Page.Validate(string) method? Based on the documentation, it looks like it may be what you want.
Page.Validate("MyValidationGroup");
if (Page.IsValid)
{
// your code here.
}
Note that the validators on the control that also caused the postback will also fire. Snip from the MSDN article...
The Validate method validates the
specified validation group. After
calling the Validate method on a
validation group, the IsValid method
will return true only if both the
specified validation group and the
validation group of the control that
caused the page to be posted to the
server are valid.