I found out minute after I posted the question, right in the help contents . I was trying things like [=""].
Manually you would enter an = sign in the field and then a) press the omit button on the status bar or b) press find and then select Show Omitted.
In a script you would A) use Perform Find, in the dialog select the field and use the = sign and then select Omit in the popup button. B) a script
Enter Find Mode (uncheck the pause box)
Set field to "=" in the calculation, the quotes are needed
Show Omitted only