If you have access to the outer surface of the cable you could measure the overall diameter with an
outside diameter measuring tape [PI Tape or equivalent].
It will be better if you can do it in 2 or 3 points and take the average. Keep a safety distance of 2-2.5 m [7-8 ft] from the termination if the cable is energized.
See:
You don't need to measure the bending cable radius but you have to check the bent radius will be not less than permissible. You may build a simple tool from a rigid plastic plate [or a piece of wood, or a shape made of a strip of steel] of 45 degrees from a circle of permissible radius. By touching the cable on the inner side of the cable curvature you can check the radius. You may pour chalk powder on this and check if the cable took the chalk on its entire bent.
