It can be difficult to tell whether a purse is a fake or not, if you know nothing about it. So, your best bet would be to take it to someone who can take a look at it and point out things that show that it's real or fake. Then, you will learn for future reference. :-)
However, I can tell you a few things that I know about that may help you out.
 If you purchased the bag on Ebay, Craigslist, at a flea-market, or similar... there is a chance it may not be real. So, to avoid accidentally buying a fake... it would be best to only purchase the bags from from actual high-end stores... i.e stores that have the same name as the brand of the bag, and quality department or boutique bag stores.
 If the price is really cheap, then it's most likely a fake. Real designer bags like Coach, Chanel, Prada, etc... are not cheap, even when they are used, because they are so well-made and have a popular brand name. So, think about how much you paid for the bag.
 Look at the stitching of the bag. Real designer bags will always have good quality stitching because the designer's want to have and maintain a good reputation. If the bag has sloppy or uneven stitching, then most likely it is a fake.
 Look at the inside, and outside of the bag for tags or labels. Most fake bags will not have a name on the inside tag. Real designer bags have a stamped or hand-stitched name on the inside tag. Also, many designers have authenticity labels on the outside of the bag as well.
 Check out the material of the bag. If it is a real designer bag, it should be made of a good quality material. If it's supposed to be leather, it should smell like leather, because real leather has a distinctive smell. If it's made of some other material besides leather, it should be strong and well stitched.
 Look at the logo. It should be spelled correctly, and look good with no mistakes.
 Look at, and feel the interior lining. Most real designer bags will be lined in a soft, silky, or leather lining. It should feel good, look good, as well as feel durable. You can rub the lining together, or feel it all over and if it sounds/feels like a trash bag, or paper... then it's most likely a fake.
 Look for a serial number, most real designer bags have a serial number, usually located on the inside of the bag. Not all real designer bags have a serial number though... so just because a bag doesn't have one, doesn't automatically make it fake. You should look at everything on the bag before determining whether or not it's a fake (as well as get it confirmed by someone who knows a lot about the subject).
 If you are buying the bag used or from some place like Ebay, Craigslist, or a flea-market... look at how worn it is. Real designer bags are made of good quality material, so they are not going to rip, tear, break, split, etc... easily. Also, look at the stitching... it should be still intact, and not beginning to fray.
I hope this helps! :-)
The label and stitching r a certain way, ur best bet is to take someone with u that knows both