When you buy our Seed/Pebble lights, they come in a bundle or spool, with no pigtails or seemingly obvious markings.
In the picture below you can see a close up photo of the wires. The middle wire is always data, no matter which way you look at the wire.
The ground/negative wire is plain/unmarked
The positive (5-12v) wire has a very faint red/pink tracer running in a spiral around the jacket - we have drawn some lines on the photos to help you identify them.
The data direction is unmarked, but if you do not succeed by putting data up the data wire in one direction, you will by using the other end!

Seed light wire identification

On the black pebbles, it is similiar - but instead of red traces, you will see little white dots for the positive/+ wire:
Black seed wire layour

How do I know which end to put the input at?

With the positive wire on the top when laid out flat, the data travels in this direction:
(note the white markings on the cable denoting the positive wire at the top - with the clear wire, the red lines would be at the top).