Here's the situation : A person has already drunk some water from a glass , and i want to say this to some other person . I am having difficulties understanding what will i say to other . Will it be "This glass has been used ".
Yes it’s correct to say:
The glass has been used because the glass was used some time in the past, is still being used now in the present and certainly may remain used in future.
You also may simply say:
The glass is already used or
The glass was already used.
Using past simple express an action finished somewhere in the past which is correct in the context.
As we can see there are many correct possibilities to express this.