Here are two shortcuts:
Network. If you know a local writer who has sold in your genre, ask for the name of her agent. If a writer is signing in a local store, drop in! (Buy a book if you can afford one.) Don't give her the synopsis of your book. Make sure she's not self, vanity or PA published.
Writer websites: If another writer in your genre has a website and a contact addy, a short, polite e-mail can help.
Dear ___, I'm an aspiring writer in search of an agent. I was wondering who represents you, as I work in the same genre. And let me say I really enjoyed
Horny Hearts--and so did my boyfriend
! Please keep up the great work!"
Hey--flattery never hurts!
And here is a SAFE place to hunt for an agent:
http://anotherealm.com/prededitors/pubagent.htm
Putting an agency name on Google with the words "scam" or "rip off" can help weed out the scammers.
Here's a legit agency:
http://www.spectrumliteraryagency.com/
They have names of writers you've heard of, sold books up on the site, links to author websites, links to booksellers.