How to differentiate listed and unlisted co?

You can differentiate by the CIN of the Company. If the CIN starts with "U" then its Unlisted and if it starts with "L", its listed.

Listed Company

A listed company means a company which is listen on different stock exchanges in India or outside India and their shares are traded on stock exchanges. For Listing company made an agreement with stock exchanges and follow the guidelines given in Listing Agreement of every exchanges.

Unlisted Company

An unlisted company means a company whose shares are not available to general public for trading and not listed to stock exchanges. An unlisted company can be private limited company or public limited company

Regards  

Avinash is right.. Simple method or u can refer to Annual return of the Companies. U'll find that on the front page

Reply provided by CS Ajay Mishra is correct.

Sec 2(23A) "listed public companies" means a public company which has any of its securities listed in any recognized stock exchange.

 You also can see Form of annual return of a company having a Share capital SCHEDULE V, PART II, from there you easily identify that whether the company is listed or not.

Each company (Indian or foreign) has a unique CIN (Corporate Identity Number).If the CIN starts with "U" then its Unlisted and if it starts with "L", its listed.