For validating user input fields in a graphical user interface, a data entry field includes a graphical validation indicator. In response to user activation of the validation indicator, such as by hovering with an on-screen pointer, a message is displayed in visual association with the data entry field. The message describes one or more expected characteristics of the data to be entered, for example that the data is numerical. As the user enters data consistent with the expected characteristics, the validation indicator is displayed with a neutral appearance, indicating that no errors have been detected. If incorrect data is detected, the validation indicator takes on an emphasized appearance, such as a higher intensity and/or an error-indicating color such as red, providing an immediate error indication to the user. An error message may also be displayed in visual association with the data entry field. The error message is preferably displayed with an emphasized appearance, and may specifically identify the error in the entered data.