There are very few fake RMs. The ones out there are very easy to spot (using lining that was never used, etc). It's not like LV where fakes are everywhere. I wouldn't worry too much, but you can always post in the authenticate this Rebecca Minkoff thread for peace of mind.
Yes, there are fakes but mostly they don't have an RM branding. They don't have logo tags or engravings on them and they have linings that are not correct as Fashion1 says. If you get a bag with no markings whatsoever that is a problem. But in general, if it says "RebeccaMinkoff" on it somewhere it is OK.
Keep in mind that is just what we have seen so far.