Yes, a higher wattage light bulb will have lower impedance and allow more current to flow (that's how a higher power device at the same voltage works).
But you cannot measure a wall outlet with any multi-meter. You have to use special probes (and maybe a special meter) so you don't electrocute yourself.
That method will not let you determine the rating of a power supply. If you have a 10A power supply, and a 20A power supply, they will both measure the same if you connect something that only draws 5A.
If it's not labelled, all you can do is SLOWLY increase the load (decrease the load impedance) until the power supply voltage begins to drop too much, or until it starts to overheat- whichever comes first. Make sure the loads you attach can dissipate the heat.