Question: Concerning "[Uranium-series dating](https://en.wikipedia.org/wiki/Uranium%E2%80%93thorium_dating)", also known as "Uranium-thorium dating". 
 Uranium is present in deposits, "typically at levels of between a few parts per billion and few parts per million by weight", according to the wikipedia article on the subject.

As an example, with carbon dating, the amount of carbon-14 originally present in any given sample is consistent with the amount of carbon-14 created by the activity of sunlight in the atmosphere.

How do we know how much uranium-234 was in any given sample of rock when it was created/deposited?

Answer: 
The field of nuclear physics has established the [radioactive decay](https://en.wikipedia.org/wiki/Radioactive_decay) series for radioactive elements (see [here](https://www.boundless.com/physics/textbooks/boundless-physics-textbook/nuclear-physics-and-radioactivity-30/radioactivity-190/radioactive-decay-series-introduction-708-1301/) as well).

Unlike [Uranium 238](https://en.wikipedia.org/wiki/Uranium-238), [Uranium 234](http://www.nuclear-power.net/nuclear-power-plant/nuclear-fuel/uranium/uranium-234/) is not [primordial nuclide](http://www.nuclear-power.net/glossary/primordial-matter/). It is a indirect decay product of Uranium 238.

By knowing what elements, and their isotopes, are present in rocks and how much of these isotopes are present, combined with the radioactive decay chain it is possible to determine the amount of Uranium 234 that was once present when the rocks formed.
**Lead and uranium have completely different electro-chemical properties**. So as David mentioned it does not matter how much uranium the sample had (as long as it is enough to measure) what matters is whether it contained any lead when it formed. Uranium and lead do not form the same kinds of bonds with other elements, so what you do is look for a uranium based crystal or uranium incorporating crystal. Crystals have a very regular bonding pattern, so you know when the crystal formed it was uranium and not lead, becasue lead cannot form those types of bonds with the smale materials uranium can. So when you find lead locked in those crystals you know it is because when it crystallized it was uranium and later decayed into lead. 

Most samples used are crystals for this reason. We pick materials that could not normally include lead, by doing this you can add up the total amount of lead and uranium and that will tell you how much uranium there was originally. **Zircon** is especially common for this becasue it will readily incorporate uranium but strongly rejects lead, so even if there is lead present in the solution (molten rock) in will not be incorporated into the crystal, because it is actively repelled by the crystal surface, thus insuring any lead you find came after the crystal was already solid and the repulsion could not push it out.