said by Jason Levine :

Except, from what I understand, they haven't been downloading and confirming these things. They just have screenshots of alleged copyright infringement. Screenshots could easily be manipulated or, as HiVolt point out, show files that aren't really infringing despite their titles.

Combine this with Media Sentry (the source of much of the RIAA's evidence) apparently acting illegally in some states and the RIAA's blunders of suing people who are obviously innocent (don't own computers, own Macs when the software allegedly used was Windows only, are dead, etc). The RIAA doesn't care so much whether they actually target copyright infringers as they want some nice big numbers in their press releases ("400 more lawsuits filed") and nice settlement money flowing back into their pockets ("settle now or our price goes up").

said by HiVolt :

What gets me, is that how can they for sure know that a shared file is an actual copyright violation. I can put 700mb worth of garbage in a text file, rename it latestharrypottermovie.avi and share it.

Also, bit torrent and most other P2P send little pieces of info to their peers. You almost never download the complete file from a single source, unless its just one seeder and you the one leecher.

How do they determine if what you're sharing is copyrighted and warrants prosecution? Do they download the entire file from you (and you only) with some concrete proof? Or is it just based on what appears listed as shared in your list, and they have no idea what the content actually is?