My local newspaper has a lifestyle section that will often include book signings taking place that week. Sometimes those include a well-known celebrity.

Check local Malls and bookshops websites for celeb apperances

Follow their Twitter.

Or join other forums/websites that post that kind of info!

Just hang around where she is filming and hope she comes over to meet you. I don't know if she is a good IP signer or not. Good Luck!

You can be near the set of a movie she's filming. If she is nice enough then she will sign autographs or take photos. You can see where they are filming on location Vacation.com. They have addresses on where they are filming. Emma is going to film Noah which is your best chance to meet her in NYC.

I'm not an expert, but I would say to just check the person's web site frequently. Usually if they have a signing, they'll make an event on their Facebook page or official Web site.
You could also check some Twitter or other social network fan accounts. A lot of people regularly post updates about celebrities.
Good luck!