1. Visual check the batteryDoes the battery have correct logos? are there any printing errors or fuzzy text? does it have a "solid" feel (the cells should not be loose), if you have a genuine battery of the same capacity to compare with, does it weigh the same? does it "snick" cleanly into the laptop?
2. Does the box have any manufacturer logosUsually a genuine battgery will come in a box/bag or packaging which has the company logos/part numbers or even matching serial number, this is not always the case, but if you have these things it's a good sign your battery is genuine.
3. Are the instructions branded?Because of the different markets fakes are less likely to come with the correct manufacturer instructions, if you have a very generic leaflet with no logos, contact details or customer support information, then this is suspect, if there are typos then the alarm bells should start ringing.
4. Does the capcity on the battery match the type of battery?Most manufacturers only produce two or three versions of batteries for a specific laptop, there are two measures here, the number of cells and the capacity, make sure that the manufacturer makes the particular configuration that's on offer. Note, capacities can be listed two ways mAh and WHr (divide WHr by the voltage to get mAh).
5. Is there a genuine serial number?Manufacturers will (almost always) have a stuck on label with the serial number of the battery, if there is nothing unique about your battery then this should raise a question.
6. Test the capacityMost operating systems supply a software battery meter, this may (e.g. Ubuntu) also give the exact capacity, also many manufacturers also can supply specific battery test tools (often given as calibration tools), this is possibly one of the best ways to identify a genuine battery.
Note, to get the best out of your battery, calibrate it, this is usually fuly charging and fully discharging (before charging again), but check the manufacturer instructions for specific details.
What do you do with a counterfeit battery?
Well, if it's sold as genuine but is not, then as with any "not as described", contact the seller in the usual way (open a case if not satisfied etc.), if you've bought the battery from overseas then a return is likely to be costly (and usually the return is at the buyers expense), in my case despite the fact that eBay found in my favour I had to pay for the return postage (which was over half the cost), if you're happy to use a "compatible" battery (be warned, it may not have been made to correct specifications), assuming it's reliable (decent capacity) you may want to consider keeping it, although, no OEM would every endorse you using a non genuine battery, and if your laptop becomes faulty within waranty they could raise the question whether the "compatible" battery has damaged the laptop which could put you on dodgy grounds.
You may find that manufacturers are interested that their products are being counterfeited, by all means take photos etc. and send the information to the genuine manufacturer, some companies even have special schemes set up to protect their name.