It is never possible to know for sure why any human being or human organization does anything. Therefore, it is not possible to know whether any given firm is being sincere in its socially responsible actions.
Any action that is socially responsible can be seen as a cynical ploy to create a false image. However, almost any socially responsible action can also be seen as a sincere attempt by the firm to achieve socially desirable goals. The only way to try to determine the firm's sincerity is to evaluate the entire body of actions that it takes. If the firm does one or two things that seem socially responsible while doing many other things that work against social goals, the socially responsible actions are probably just marketing ploys. If, however, the firm consistently takes responsible actions, it is probably sincere.
Even this, however, is in the eye of the beholder. It will never be possible to objectively answer this question simply because human motivations are exceedingly hard to identify.