You could make a really big IF statement:
IsEmpty ( FirstField )
IsEmpty ( SecondfField )
"You forgot to fill out a field!"
This would be put as a calculation in a 21st field.
But I don't know if that's the classiest way to do this.
The only problem is that now, this will always show, even when someone starts filling out the form.
Another way to only do the check at the end is to set a script when your button gets clicked after filling out the script.
Then the IF gets put in a script. If any of the fields is empty you set the 21st field to the value "You forgot to fill out a field."
And you halt the script.
Otherwise the script continues.