How can you tell if you're flagged for an audit?

and please don't just say you get a letter. I found this website that offers "audit monitoring", supposedly giving you an alert when the irs computer systems flag your return, supposedly months ahead of an actual audit. I figure it might be similar to getting irs transcripts. There are services that offer this, but you can just do it yourself through the irs website. It's pretty easy. But this audit flag thing, how can we know if a return has been flagged?