The attched application demonstrates 2 methods of doing so.
The "No Lowercase" tab, compares (using the Text/Equal template) the original input to an upper case version of it (using Text/Upper Case).
The "Only Uppercase" tab, compares (using Text/Match) the input to an appropriate.
Of-course these 2 methods are not identical in their results (the first allowing non-alphabetical chars).
Both examples use an <On Change> event on the input field to perform the comparison, and using a <Validation Message> element (see here for more details).
For best results, use the Firefox browser..