www.tineye.com
Pretty great site, just copy and paste the link into the box and in seconds you can see where stuff has been around the web.
This one only came up with one other site
http://www.tineye.com/search/5bf2d4db8f9132b0386bd8d32ed631c65c9d535d/
but still it's fairly easy to see
http://www.tineye.com/search/69f9322f86125ed18a11e65441b0b09d0a8acd83/
http://photoshopcontest.com/view-entry/205555/the-rising.html
example of one of mine...
I had one contest where I used a great outside source and didn't list it, it was winning for a while on the weekend and caught a ton of flak when someone found the source for it. So from then on... I listed my outside sources if I felt it was necessary.
http://photoshopcontest.com/view-entry/99848/anticipation.html
and all I really did was took the escalator from another photo and used the picture from the chop... I still integrated the reflections and color corrected into the main source, but I guess people thought I did an all source because I had no notes at all... A case where even not trying to fool people, but still did so...
As much as I like TinEye, it doesn't recognize the above chop, but someone knew the source and called me out. So it happens to the best of us.
http://www.tineye.com/search/c055da91f874bd0cfae4ee15dc689bbfdbfda27d/
Anyway... TINEYE is where I check all of his work (and any others I feel may not be forthcoming with outside sources) before voting...
unfortunately this was a anon contest.
At least he's pretty good at cut & paste