Null string check

Hi..

I would like to check if user input something in text filed (single line/multi line ) or not.
IF user bypass the text filed during data entry by pressing enter or tab key and jump to next filed alert them to enter something in text filed. I know there is check mark in filed property which alerts them but that come all the way to end when you press submit button. is there any way we can check "null string" "enter key" or "tab key" or no entry in text filed and alert them ?

Thanks,