That's what people mostly asked to know the time accurately. Many watch buyers always have a great deal of misconception about what time it is really.
If you want to test your watch, you can set it with
nuclear clock. Nothing can make it really accurate without second-hand watch. That is the most accurate time on earth but few of us doesn't have it. Don't make it worry, you can own it by download in the nuclearclock websites or you can get very accurate times from a high quality quartz watch.It's common that a quartz (battery-powered) watch is probably more accurate than you will need. The quartz crystals that drive these watches vibrates at a frequency so fast that you will never be able to detect any fluctuations in them. However if your watch is mechanical, its accuracy can vary quite dramatically. This depends on a lot on how evenly the spring that the powers movement is unwounded. Remember that in any watch, the closer you get to the end of the power reserve, the less accurate the watch gets. That being said, some watches like the IWC Portuguese Automatic, will actually show your power reserve at a shorter time than actual just to ensure that you rewind your watch before it losses accuracy.
You can note how fast or slow your watch runs each day. If results are fairly constant (i.e. your watch runs an average of 15 seconds slow a day), you can bring it to an authorized service center to have it regulated. Bear in mind that the lubrication in any new watch must be given time to settle before you get peak accuracy so don't rush out too quickly.
When it comes to buy watches, one guarantee of accuracy is to look for official chronometer certification. This represents the pinnacle of accuracy among mechanical watches. A watch will only be granted a certificate if it has a mean daily variation rate of between -3 to +6 second per day in different positions and temperature over period of 15 days. The certificate contains the actual rate results in every position to prove the new owner that it has passed this stringent exam. The certificate is given by the COSC (Controle Official Suisse des Chronometres) a neutral organization headquartered in Switzerland that is charged with examining watches and issuing the tittle of official Swiss "Chronometers".
In recent years, companies as Rolex, Breitling, Panerai, TAG Heuer and Corum have perpetually been among the top eight in receiving chronometer certification. See more about product information on Amazon.com. However, some companies actually don't bring their watches for certification viewing as a marketing decision because a watch with no certificate doesn't mean it is not accurate but just merely has not submitted to the COSC.