The only way I am aware is to manually determine if the current text is too wide.
Add a tooltip to the form. Then handle the TextChanging event on your combo box.
private void ultraCombo1_TextChanged( object sender, EventArgs e )
{
var textWidth = TextRenderer.MeasureText( ultraCombo1.Text, ultraCombo1.Font ).Width;
var textBoxWidth = ultraCombo1.ClientRectangle.Width - SystemInformation.VerticalScrollBarWidth;
if ( textWidth < textBoxWidth )
toolTip1.SetToolTip( ultraCombo1, "" );
else
toolTip1.SetToolTip( ultraCombo1, ultraCombo1.Text);
}