Simples Assim

import net.rim.device.api.ui.Field;
import net.rim.device.api.ui.FieldChangeListener;
import net.rim.device.api.ui.component.BasicEditField;
import net.rim.device.api.ui.component.ButtonField;
import net.rim.device.api.ui.component.Dialog;
import net.rim.device.api.ui.container.FullScreen;

public final class SampleScreen extends FullScreen {
  private Field btn;

  private Field txt;

  public SampleScreen(final long style) {
    txt = new BasicEditField() {

      public boolean isDataValid() {
        return getTextLength() > 0; // Can't be empty
      }

    };

    btn = new ButtonField("Validate");

    btn.setChangeListener(new FieldChangeListener() {

      public void fieldChanged(final Field field, final int context) {

        if (txt.isDataValid()) {
          ...
        } else {
          Dialog.alert("This field is required!");
        }

      }

    });
  }

}

http://www.blackberry.com/developers/docs/4.6.0api/net/rim/device/api/ui/Field.html#isDataValid()