Whistleblowing. The disclosure by a person, usually an employee in a government agency or private enterprise, to the public or to those in authority, of mismanagement, corruption, illegality, or some other wrongdoing. Since the 1960s, the public value of whistle-blowing has been increasingly recognized.
The above is the meaning for whistle blowing given by google.
Now you want to whether you can blow the whistle when you do not have substantial evidence to prove the incident which you want to establish by blowing the whistle.(?)
Well, there is nothing wrong in keeping quite without informing the authorities about th incidences happening before your eyes or believed to go going on behind the screen when you feel that you cannot be able prove it when you are required to do it.
In all the probabilities, under such a situation, your presence silence may help you gather more information and evidence to blow the whistle loudly at an appropriate time which would be effective and the purpose can be achieved.