How can I tell my form is being used in design time and not runtime

Check the the property Form.DesignMode.

But, note that when in Visual Inheritance mode (designing a derived form), your Control’s DesignMode property will be true when the base form’s constructor gets executed in the design-time.

To workaround this, you could check if the app in which your control is running is not devenv.exe, as follows:

string exePath = Application.ExecutablePath;
exePath = exePath.ToLower();
if(Application.ExecutablePath.ToLower().IndexOf('devenv.exe') > -1)
{
	// Then you are running in vs.net.
}