I have a small PSU board out of a defunct DVD player that I'd like to use as a supply for my breadboards. It has two rails, +12 and +5 with two GND lines, but there's no current indication. My guess is that it's in the 500mA-1A range, but I really have no clue. What's the best way to test it and determine a safe limit? Also, if it requires a load in order to operate, should I just wire in a big resistor across +12 to ground?
The case for home appliances usually has the power consumption written on it. That will give you an upper bound if you assume say 80% efficiency.
You could hook a fat power resistor up chosen to dissipate say 1A and see if the output voltage drops indicating overload.
Really, you could do better with a dual-rail PSU from some obsolete computer equipment (eg Zip drive), available really cheaply at flea markets etc. If you only want 5v, old mobile phone chargers are good; prefer switchmode (light, efficient, well regulated) over transformer (heavy, inefficient, poor regulation).
Is this an open frame kind of supply? If so, can you make out what kind of regulation it is? Linear supplies tend to have a single rather large transformer, while switchers usually have several, smaller inductors.
If it happens to be a linear type supply, the capacity would be limited by the regulator devices, e.g., if there were a 7805, you could get a data sheet for the 7805 in the particular case style they used.
If it's a switching supply (more likely, really) there might be other ways to tell, but I'm not personally familiar enough with switchers to be able to say.
If you have some power resistors already, it should be safe to load the power supply with them, starting with a fairly high resistance and slowly decreasing the resistance (increasing current) until you see the output voltage starting to drop too far below its nominal output. The output is probably rated for +/- 5% or 10%, so when the output falls below 90% of nominal voltage it's overloaded. You can go in small steps and plot a graph if you care about the details.
If you change the resistance in too large a step you might go too far above the rated output, which could cause damage but is more likely to blow a fuse, so be careful of that, especially if the fuse doesn't look replaceable or you can't tell which component is the fuse.