I think the answer varies depending on how each individual defines success.
A general definition of success might be achieving a goal or objective. It stands to reason that you know you’ve achieved success once the goal or objective is reached.
Unfortunately, it seems once a particular goal or objective is achieved people often feel the need to modify their goal or create another goal or objective. For these people “success” is always a “work in progress”. So maybe success may be more accurately defined as achieving a goal or objective to fill or satisfy a perceived lack or inadequacy.
For me, success is my ability to accept who I am….where I am…here and now.
I know whether or not I’m successful via my level of contentment or happiness at any given moment.
So if I become aware that I’m angry or frustrated, I know that I’m temporarily unwilling to accept who I am…where I am…here and now.
A lifelong work in progress.