If you really really look closely, with back lighting, you can "find" the cut points.
The makers put bulbs together in series, adding the voltage up to the rope voltage. You can actually see a "cross wire" from bulb to bulb that connects them in series. When you get to a Cut Point, there is no cross wire. Just the two bus wires (Hot and Common). You cut exactly between the two bulbs that don't have the cross wire.
If they used 5volt bulbs, then they would tie together 24 bulbs to equal 120 volts
If they used 6volt bulbs, it would be 20 bulbs to equal 120 volts.
A common value is the 3.3volt bulb. That gives you 36 bulbs, at 1/2" spacing, for an 18 inch cut space.
Just depends on the maker.
If the rope is new and clean, it is pretty obvious. If you're lucky, you'll also see the scissor printed there.
If you are looking while power is applied, PLEASE unplug before cutting. 