Checking the PasswordStrength in Codebehind

I have a PasswordStrength indicator on my form, but I want to check it's status when a user clicks a button to change their password. It should not allow them to update the password until it is above Good Strength.

<asp:TextBox ID="tbPassword" runat="server" BackColor="PaleGoldenrod" MaxLength="55"
                                        TextMode="Password" Width="175px" Height="22px"></asp:TextBox><ajaxToolkit:PasswordStrength ID="PasswordStrength1" TargetControlID="tbPassword"
                                        StrengthIndicatorType="BarIndicator" PrefixText="Password Strength:" HelpStatusLabelID="Label1"
                                        PreferredPasswordLength="8" MinimumNumericCharacters="1" MinimumSymbolCharacters="1"
                                        BarBorderCssClass="BarBorder" TextStrengthDescriptionStyles="VeryPoorStrength;WeakStrength;AverageStrength;GoodStrength;ExcellentStrength"
                                        runat="server" TextStrengthDescriptions="Very Poor Strength;Weak Strength;Average Strength;Good Strength;Excellent Strength"></ajaxToolkit:PasswordStrength><asp:ImageButton ID="btUpdate" runat="server" ValidationGroup="ValidationGroup5"
                                        Visible="False" ImageUrl="~/Images/UPDATEblue-button.png" ImageAlign="AbsMiddle"
                                        Height="23px" Width="58px" />

I have yet to find a property of the control that I can check on the codebehind. Any help?