TabControl supports selectedIndexChaged event. You can try this:
private void tabControl1_SelectedIndexChanged(object sender, EventArgs e)
{
if (tabControl1.SelectedTab == tabPage1)
toolTip1.SetToolTip(tabControl1, "TabPage1");
else if (tabControl1.SelectedTab == tabPage1)
toolTip1.SetToolTip(tabControl1, "TabPage2");
}
But perhaps you shoud consider to assign tooltip value for each individual tab page. This way you don't need to do any coding.