How do I find out what PSU I have?

There is no way other than opening the case and reading the label on the PSU. If you have a prebuilt like HP for example, the online product page may list the PSU specifications for you particular computer model.

well another way to look into this, what is the reason that you what to know what PSU you have? If you do have a factory computer and your looking to add components, such as a videocard, or extra hard drives, the factory ones dont put much more than what the stock components need in there. So if the situation is something like that, you may want to factor in getting another PSU. But if you really just want to know without opening the case there is this product that is called kill-a-watt. http://www.xoxide.com/p3-kill-a-watt.html It is a monitor that you plug in between any device and the wall. It monitors how much electricity something draws. This will give you a reading on how much power your computer actualy uses and i think thats the best way without a screwdriver not knowing the manufacturer and model.