The documentation indicates
"If you change parfor to for, the use of s outside the reduction assignment relies on the iterations being performed in a particular order. In a parfor-loop, it matters that the loop "does not care" about the value of a reduction variable as it goes along. It is only after the loop that the reduction value becomes usable."
so there is nothing you can check in the loop to determine whether you should continue. Though I guess you could write to a file and check the file.
Cleaner would be to parfeval() and as the results come back, when you finally get a true, cancel the remaining futures.
