Question

How we know that which is strong acid and weak acid without an indicator?

Answer 1

A solution of a weak acid cannot be titrated with a weak base using an indicator to find the end-point because the pH change is too gradual close to the equivalence point.An indicator works when the solution's pH is in its range. It's not necessarily the equivalence point. The point is called the end point where it actually changes color.

If the difference in volume is small between the equivalence and end points, then the indicator can be used. Since a gradual change implies more volume must be added to obtain the same pH change in other acid base titration, the difference between equivalence point and end point(if you use indicator) will be more and therefore indicator will not be suitable for it.