"Don't launch with a crappy product -- launch as soon as what you have is better than what is out there." If I remember correctly, that is advice given by Paul Buchheit.
e.g. Hearing someone talking about fear is entirelly different from living it on your own.
The same goes for many stages in a startup. If you don't experience it, any advice in the world will not matter.
I wouldn't be here if I thought I knew everything, but If you/I expect someone else to tell you how to understand if your product is ready then you lost already.
For example if you want me to be more precise.
In the certain advice everything is obselete and relative.
What is the competition? Have they already launched? How easy will be for them to just replicate you and crush you? When do you know you are better from the competition? What would make sure you will avoid crushing you?
All these questions relative to that advice are so crucial that makes the advice broad enough unless you are already experiencing that pressure and already have the realization of the market you are trying to enter etc.
Thats why advices are meaningless until you experience them.
You need to value the essence of the advice, not always upon who gives it.
You will get many good advices from many people if you don't undervalue them.
the advice is not as important by who is said, as much if the person giving it is identifying what to tell you relative to your current experience. Otherwise any other advice will not matter.