How do I know that someone isn't sending out viruses on BitTorrent?

In short, you don't. You should treat something downloaded with BitTorrent just like any file downloaded from the internet - that is, if you don't trust the source of the file, then you should use caution when opening it. If the torrent site you obtained it from offers comments, be sure to read those first. But regardless of the comments, running a virus scan on the downloaded files is usually a good idea. µTorrent guarantees that the content you download is not altered from when the torrent was originally created, but if the source files used to create the torrent were already infected, this will provide no protection!