If you want "real time" checking I believe you would need to use JavaScript.
Otherwise you could check for it on postback and convert lowercase characters
to uppercase behind the scenes. It depends on what you want to accomplish.
I'd like for once the user enters data in the text field for the function to
automatically convert to uppercase once the user leaves the field. Sounds
like JavaScript or does the .Net framework have something for this?
At least as of the 2.0 framework there wasn't anything like that that I know
of. It's probably easiest to do this with JavaScript (using OnKeyPress, or
some such).