Hello
The stock market value of any stock market quoted company can be checked in various places.
The link below is from the Financial Times' website
FT.com / Markets data / DTG:LSE: Performance for Dart Group
Of course, the stock-ex value of a firm does not always match the value of a firms assets. If a another firm was looking to take over a company who has shares listed on the stock market, those shares will rise, and thus the value of the company will also rise.
There are many companies quoted on the stock market with large financial values, yet some of these firms own very little in the way of assets. Their prime asset could be anything from an "idea in development" (such as a new medicine) to their own brand name.