The most straightforward way to determine these things (if you can trust the manufacturer not to lie) is to look at the PSU label.
Here is a webpage that shows the label for the Cooler Master 460.
I will just assume for the sake of argument that the Cooler Master manufacturer doesn't lie on its labels.
http://www.coolermaster.com/product.php?product_id=3686
If you look at the RS-460-PCAR-A3 label, it shows
3.3v = 22a
5v = 25a
That means the most you can draw off of the 3.3v at once is 3.3v x 22 = 72.6w
On the 5v you can get 5 x 25 = 125w
If you add both of those together you get just about 200w in total (197.6w). However, under those you see a box that covers both spaces that says 165w on it. This means that both of these are "tied" together, so to speak, and the max you can get from both of them in total is 165. However you connect items to these power things it can't use more than 165 out of the theoretical 197.6w maximum.
Moving over to the 12v lines, the most important type for computers in 2011 you see that there are 2 listings and both say 18a. Each of them can theoretically do 18 x 12 = 216w. Totalled together that would be 432w. However, you see a box right below them that shows them linked together with a maximum usage of 312W. That means that 120w of the theoretical potential of the wires can never be realized.
If you go above the combined figures from the bottom row with the total of the draw from the individual lines, then the PSU will, you hope, just shut itself down to protect the computer. It is also possible, especially for cheapy PSUs, that a part literally melts and a boatload of current flows through the wire to whatever is on the other side and melts that too (like a video card).
There are a whole slew of protective features that have been invented to help protect parts from being damaged by power. Most of them aren't used by cheapy PSU makers, leaving all the risk in the hands of the PSU users.
Going back to the power thing from before....
To some extent it is going to be trial and error to find out if you are overloading some wires.
If you are trying to get more than 312 in total out of the 12vs, you are overloading it. That part is pretty easy.
The thing about it, though, is that each of those 18as on the 12vs can be overloaded individually. There are two "halves" to the power delivery and each one can only do a max of 216.
A high end processor is about 125w. Even low end or mid range ones are 65 or 95w. All of that figure goes on one rail. Then you have a video card that may pull another 200 itself. If it uses the motherboard slot (75w) and 2 x 6 pin PCIE connectors (2 x 75w) that is 225w.
As you can see, the 225w can't fit on 12v x 18a (216). The only way to get a card like this to work would be to have the motherboard slot pulling from one rail of 18 while the two PCIEs pull from the other rail of 18.
That would leave 75 on one side and 150w on the other side, both of which are under 216 so theoretically OK.
That being said, a 125w processor and a 225w video card would be 350 in total and it would blow right through the 312 max output for the 12vs x 2 so any time this theoretical system drew max power it would either shut down or start melting parts.
It is also possible that the manufacturer lied and wrote 18a when they really meant 15a and they wrote 312 w total when they really meant 250w total. You would never know this and you could dutifully hook up things thinking the label was accurate and potentially destroy your parts as a result.
These considerations are why people here always advise to stick with certain brands (known not to lie, or to even label power conservatively) and why single rail designs are easier to configure than multi-rail designs.
If the 165w and 312w (separate rails, overloadable individually) were just one bar that said 477w on it (single rail), then you could take more juice out of the 12vs and less out of the 3.3s and have a PSU that still works just fine.
In a practical sense, you can only really play around with the power connections and try to re-engineer the setup and then try to turn it on again and see if it works the next time.
So you could just start taking power cords out of the back of, say, your hard drive and then replace them with power cables on a different wire and try to turn it on.
If you are having load balancing problems, that could fix it.