It's not easy to make judge about any medicine because each drug has certain thermal stability . but can make simple spectroscopic test like UV-Vis ,FT-IR to monitored any change in the active site or functional groups presence. 

the amount of drug in any formulation has to meet specific criteria to achieve optimal efficacy without overt toxicity. As with any chemical, medicines will react to the environment start and degrading with resultant loss of activity. The expiry date for any drug is defined by the amount of time it takes for the amount of active drug in the formulation to drop below the recommended range required to achieve the required (optimal) efficacy - of course when stored within the recommended storage conditions.

These are manufacturing and testing specifications, including upper and lower limits for the amount of the active drug in each dose. You can find lots of reading material online, but here's a link to a good concise start:

http://www.dsls.usra.edu/education/grandrounds/archive/2009/20091027/khan.pdf

hope this gets you started. Good luck!

The expiry date is that point in time when a pharmaceutical product is no longer within acceptable specifications for potency and stability. In determining an expiry date, a range of characteristics of the product are studied over time. One important characteristic is the chemical stability of the active ingredient. Temperature has a pronounced effect on the rate of degradation of the active ingredient. As the rate of decomposition usually doubles for every 100C rise in temperature, it is important to comply with the storage conditions specified on the container. The expiry date may be set as a fixed time after manufacture, dispensing or opening of the manufacturer's container.

I think these links could be useful for you.

http://apps.who.int/medicinedocs/en/d/Jh1813e/3.3.3.html

http://www.gmp-compliance.org/guidemgr/files/1-7-3.PDF

Thanks to all

Thanks  Nataliya Rybalka   Mind you the expiratory date is not specific to the drugs but all  material  e.g. food has  expiratory date

Expiration dates are set on a particular drug by using the accelerated storage test. Since the rate of disintegration of ;most drugs are very small at room temperature. The rate of disintegration is determined at higher temperature and extrapolated back to room temperature. Knowing the rate helps to calculate how long it can stay on the shelf before it loses certain percentage as prescribed by the regulating bodies.