If radiobutton is checked , AND text box has info... make button active?

I have a radio button group that is a yes/no question.  If the user selects the yes radiobutton of the group, a text field becomes enabled for them to enter corresponding info.  If the user selects the no radiobutton, the text field says disabled and a continue button is enabled allowing the user to continue.  That all works great. 
The problem I can't figure out how to make sure that if ther user selects the yes radiobutton, that the continue button only becomce enabled if the user also enters text in the corresponding field.  (Remember, the text field isn't enabled if the user clicks no).
Here's what I've got in my code...   
<asp:RadioButtonList ID="rbMultipleClients" runat="server" AutoPostBack="true" OnSelectedIndexChanged="rbMultipleClients_SelectedIndexChanged" ...