The logic here would be to loop through all the input controls and see if values exists for them. If any one of them is still empty, don't enable the submit button.
There can be few ways to do it. If you have just the 'input' control for taking data then just loop through all of them. Or else go control by control.
example:
Using normal Javascript:
function Validate()
{
var inputField1 = document.getElementById("<%=inputField1.ClientID%>");
var inputField2 = document.getElementById("<%=inputField2.ClientID%>");
if(inputField1.value.length > 0 && inputField2.value.length > 0)
document.getElementById("<%=btnSubmit.ClientID%>").disabled = false;
else
document.getElementById("<%=btnSubmit.ClientID%>").disabled = true;
}
CodeBehind file:
inputField1.Attributes.Add("onblur", "Validate()");
inputField2.Attributes.Add("onblur", "Validate()");
This should do.
P.S.: There is way to make it easy using jQuery but that would need a little knowledge about it.
P.S.2: There are validator controls too that does it but current solution is based on as asked.