I'm guessing the timestamp, in its simplest form would just submit the time when the page was loaded as a hidden variable in the form, and compare it to the tim

I'm guessing the timestamp, in its simplest form would just submit the time when the page was loaded as a hidden variable in the form, and compare it to the time the form was submitted. If it's less than something reasonable for a person (say, 20 seconds or something), then it was clearly auto-filled. With a little help from javascript, you could even expand this to the individual fields. As someone mentioned in the comments above, it would be pretty trivial for spammers to adapt to this if they thought it was common, with a few random pauses. Perhaps they already have...