The U-2 has a rear view mirror on a long stick. It's there to check if the thing is laying a contrail or not.
Rest assured, that if it were possible, by planning alone, to ensure a 'negative contrail situation', it would be done.
The presence of the mirror shows that it just cannot be reliably predicted whether or not a contrail will form.
All that matters for the U-2 driver, is that he can see if he's pulling one, and do something about it if it's giving his game away.