Well, vietdraz, you stated in one of your replies, or maybe the original statement, that there was a light that used to be on all the time and no longer is. To add to that, many newer motherboards have a light on them that stays on as long as there is power to the machine(even if the power is off, as long as the PSU is getting power and working correctly) If your board has such a light and it is off, then thats a good indication that the PSU is dead. Don't go buy a new one just yet, you say you are OK with tearing things apart, so, if you have not yet located a fuse or breaker on the PSU, disconnect the power cable from the PSU, remove it from the computer's case and open it up(be careful, as some are difficult to open and you don't want to damage anything, there are also a couple of capacitors in the PSU that can give you a good jolt if they still have power stored in them) Look around inside the PSU for a fuse. It should be a glass type fuse that may be in a standard type fuse holder, or it could be soldered in place in some PSUs. Look at the fuse, see if it is blown, also look around and see if you see any obvious signs of damage. If the fuse is blown, simply replace it(or have it replaced if it requires soldering and you aren't comfortable with doing that)
Once you've checked the fuse and it looks ok, or if you notice something ungood inside the PSU, then it is likely that you need a new one.