You look at hundreds if not thousands over your working life time and you recognise what size a cable is. Failing that you can measure it. (Bear in mind that cables are defined by their cross-sectional area, not the diameter)
I use three crimp terminals where the size is not written on cable, red is 1.5 mm sq so tight fit 1.5 and loose 0.75 or 1 mm sq, the blue is 2.5 mm sq and the yellow tight 6 mm sq and loose 4 mm sq, this assumes metric cable.
With 14/0.012 cable not as easy, again use terminals as go/ no go gauges. If you go on to cable manufacturers page they do give some sizes, so you can measure the outside of a cable and compare, however not all the same insulation thickness.
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