Welcome to TA's hangout helloguy
Out of curosity which team do you belong to?
Btw defective RAM can also scramble WU's with the same effect ,the WU is finished very early (can be a matter of seconds if its really bad!:Q).As long huge amounts of WU's aren't put out in this fashion its not a problem for SETI as they resend each WU 2-3 times to check for errors like these.
To answer your original question more precisely ,the way you tell you have a defective WU is that they finish exceptionally fast ,eg my XP rig does the fastest WU's in about 2hrs 55mins ,if it goes much below that then I either had a defective WU (man made interference in it for example) or my rig is goin FUBAR! 😉