Many of these are quite simple. If you have a very thin piece of metal, with a flat tip, you can fit it down between the wheel, and there will be a flat spot. Line them all up so the flat spot is up. If this doesn't open, then rotate all dials around one position, until it opens. Sometimes the first of last number doesn't have this flat spot.

For many cheap padlocks like this, I can pull out on the shackle a bit, which turning the wheels, and feel when it has some resistance. The few times I've done it, I've opened up a lock in a minute or two.