If I have a contact poison, can I throw it on someone to affect them? Contact means creating contact with someone, but I didn't find an answer about how contact is made. Is it for the DM to decide or is throwing contact poisons unavailable?
Rules as written, the standard way to deliver them in combat is by using them as an injury poison. They're clearly designed for out of combat intrigue arbitrated by the GM.
The Poison Vial of Distance allows ranged delivery of contact or ingestion poisons.
Favored by assassins, this magical vial allows the application of poisons at a range of up to 30 feet. The poison must be supplied, and must be a contact or ingested poison. The user adds the poison to the vial and then shakes it in the direction of the item or creature she wishes to affect, making a ranged touch attack (a poison vial of distance provides a +4 circumstance bonus on this roll). If the attack is successful, the poison is successfully delivered to the object or creature intended.
Vials are used throughout the rules to deliver other "contact" substances like acid and holy water, so using contact poison in mundane vials as splash weapons is reasonable but is not explicitly allowed, so talk to your GM.
If you want to simply dump contact poison on an enemy, Holy Water provides an example of how that could work.
A flask breaks if thrown against the body of a corporeal creature, but to use it against an incorporeal creature, you must open the flask and pour the holy water out onto the target. Thus, you can douse an incorporeal creature with holy water only if you are adjacent to it. Doing so is a ranged touch attack that does not provoke attacks of opportunity.
This isn't explicitly allowed with poisons, so you should discuss it with your GM.
There is no reason you could not throw a contact poison, especially since some creatures spit their poison.
If it touches bare skin, or an injured part of their body, yes, the contact poison would be effective.
Contact: These poisons are delivered the moment a creature touches the poison with its bare skin. Such poisons can be used as injury poisons. Contact poisons usually have an onset time of 1 minute and a frequency of 1 minute.
Creative Applications of Delivery
First of all, the key line here is: Such poisons can be used as injury poisons. So could you place contact poison on a weapon and attack them with that weapon? Yes. It just so happens contact poison is so potent, that it doesn't require an injury.
A good example on television, was Elleria Sand poisoning Myrcella with a kiss. In this case, she applied deadly contact poison to her lips, and when kissing her mark, poisoned that person.
Another example on film is the dilophosaurus in Jurassic Park, spitting is contact poison onto its victim. They even state in the movie it was a paralytic poison that would commonly be spit into the eyes.
A good, albeit very morbid, example in reality, was the late Richard Kuklinski (hired killer) that would place cyanide in a drink, and 'accidently' bump into his mark, spilling the contents all over his victim. The deadly contact poison was absorbed into the skin and killed them.
The other answers also provide good methods of delivery.
But, think of contact poison as the ultimate assassin incognito weapon. It could be very hard to trace because there doesn't need to be a wound present. It would be very discreet, because to apply it requires only contact.