It depends on what information you have.
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You measure them.
s diameter of the circle in the drawing above is the segment
Not if the whole segment is inside the circle.
A line segment that divides a circle in two is a chord.If the two parts of the circle are equal, the line segment is a diameter.See 'The Parts of the Circle' in Sources and related links, below.
Such a line segment would be a radius of the circle.