Determine the maximum load of a power supply
Most modern power supplies are not damaged by loading them more than intended.  They will usually go into current limiting, shut down for a while, or at worst, blow a fuse.  Some high volume OEM supplies designed for a specific use might blow up, but encountering one of them is unlikely.  If you do, oh well, you're not worse off than before since you never actually had a power supply you could reliably use anyway.

Therefore, start at a relatively low load and keep increasing it (meaning drawing higher current or connecting lower resistance).  While you do this watch the output on a scope.  At some point it will start to sag or the supply will otherwise obviously object, like shutting down for a short while.  Since the supply will be designed to the worst case, you will see this limiting behavior at a bit higher load that whatever the rating really is.  I'd probably back off 20-25% or so and consider that the maximum rating.  For example, if things start acting flaky at 10 A, then consider 7.5 or 8 A the maximum for your purposes.

If the supply has multiple output voltages then it gets more tricky.  However, from the looks of it, your supplies have a single output.  I'm guessing that each pair of wires (red and black) at the output are actually tied together.

From the general look and size of these supplies and the size of the output wires, I'm guessing around 100 W each.